what is $<TARGET_FILE_DIR>

Usage
1 
		"$<TARGET_FILE_DIR:datachannel>/datachannel.dll"
		$<TARGET_FILE_DIR:datachannel-copy-paste-offerer>

What does it mean?
where can I find the document?

I try to use message to print the variable , but it can not get my expect result

message($<TARGET_FILE_DIR:datachannel-copy-paste-offerer>)

it just print

$<TARGET_FILE_DIR:datachannel-copy-paste-offerer>
2

Constructs of the form$<...> are Generator Expressions. They are typically there to access information that is not available at configure time; See for example this stackoverflow question for ways of determining their value.