What is ?= in Makefile

Question

KDIR ?= $(shell uname -r)

What is the meaning of ?=?

I have understood the difference between :=, += and

Answer 1

Thanks to Simon and R.T. for their quick and correct response.

Also, I have found the GNU manual that explains everything in detail: http://www.gnu.org/software/make/manual/html_node/Setting.html