P is a concept introduced in Go1.1, initially equal to the number of CPU cores by default. P is just a logical concept and has nothing to do with CPU.

The notes to P in source code go/1.13.4/libexec/src/runtime/proc.go are as follows:

// P - processor, a resource that is required to execute Go code.
// M must have an associated P to execute Go code, however it can be
// blocked or in a syscall w/o an associated P.

Then through the source code go/1.13.4/libexec/src/runtime/runtime2.go, we can briefly understand the state change of P in the scheduling process:

const (
    // P status
    _Pidle = iota
    _Prunning
    _Psyscall
    _Pgcstop
   
    // _Pdead means a P is no longer used (GOMAXPROCS shrank). We
    // reuse Ps if GOMAXPROCS increases. A dead P is mostly
    // stripped of its resources, though a few things remain
    // (e.g., trace buffers).
    _Pdead
)

The state of P can be controlled by adjusting GOMAXPROCS, which is specially illustrated by "Pdead".

From the structure field of P, you can learn more about the resources stored in P. source code go/1.13.4/libexec/src/runtime/runtime2.go:

type p struct {
        id          int32
        status      uint32 // one of pidle/prunning/...
        link        puintptr
        schedtick   uint32     // incremented on every scheduler call
        syscalltick uint32     // incremented on every system call
        sysmontick  sysmontick // last tick observed by sysmon
        m           muintptr   // back-link to associated m (nil if idle)
        mcache      *mcache
        raceprocctx uintptr

        deferpool    [5][]*_defer // pool of available defer structs of different sizes (see panic.go)
        deferpoolbuf [5][32]*_defer

        // Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.
        goidcache    uint64
        goidcacheend uint64

        // Queue of runnable goroutines. Accessed without lock.
        runqhead uint32
        runqtail uint32
        runq     [256]guintptr
        // runnext, if non-nil, is a runnable G that was ready'd by
        // the current G and should be run next instead of what's in
        // runq if there's time remaining in the running G's time
        // slice. It will inherit the time left in the current time
        // slice. If a set of goroutines is locked in a
        // communicate-and-wait pattern, this schedules that set as a
        // unit and eliminates the (potentially large) scheduling
        // latency that otherwise arises from adding the ready'd
        // goroutines to the end of the run queue.
        runnext guintptr

        // Available G's (status == Gdead)
        gFree struct {
                gList
                n int32
        }

        sudogcache []*sudog
        sudogbuf   [128]*sudog

        tracebuf traceBufPtr
         // traceSweep indicates the sweep events should be traced.
        // This is used to defer the sweep start event until a span
        // has actually been swept.
        traceSweep bool
        // traceSwept and traceReclaimed track the number of bytes
        // swept and reclaimed by sweeping in the current sweep loop.
        traceSwept, traceReclaimed uintptr

        palloc persistentAlloc // per-P to avoid mutex

        _ uint32 // Alignment for atomic fields below

        // Per-P GC state
        gcAssistTime         int64    // Nanoseconds in assistAlloc
        gcFractionalMarkTime int64    // Nanoseconds in fractional mark worker (atomic)
        gcBgMarkWorker       guintptr // (atomic)
        gcMarkWorkerMode     gcMarkWorkerMode

        // gcMarkWorkerStartTime is the nanotime() at which this mark
        // worker started.
        gcMarkWorkerStartTime int64

        // gcw is this P's GC work buffer cache. The work buffer is
        // filled by write barriers, drained by mutator assists, and
        // disposed on certain GC state transitions.
        gcw gcWork

        // wbBuf is this P's GC write barrier buffer.
        //
        // TODO: Consider caching this in the running G.
        wbBuf wbBuf

        runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point

        pad cpu.CacheLinePad
}

In the field, mcache will be explained separately. Source code go/1.13.4/libexec/src/runtime/mcache.go:

// Per-thread (in Go, per-P) cache for small objects.
// No locking needed because it is per-thread (per-P).
//
// mcaches are allocated from non-GC'd memory, so any heap pointers
// must be specially handled.
//
//go:notinheap

mcache reflects the strategy of creating objects: small objects.