In local.conf ，we find word “virtual”。
for example:

ASSUME_PROVIDED = "virtual/${TARGET_PREFIX}gcc virtual/libc"

and use "bitbake virtual/kernel“ to build a kernel。

what about these ”virtual“ mean？

A virtual dependency is one that is fulfilled by more than one alternative provider.

Example: We have lots of kernels in OE for different models. All our kernels provide "virtual/kernel". Now external module packages don't need to know _what_ kernel they're building against, however they need that _a_ kernel has been built - hence they rely on 'virtual/kernel'.

This is like saying "I don't care about which kind of ice-cream, I just want ice-cream"