This is a full specification of a dependency:
3 is a task ID.
FS is a dependency type.
P0D is a lag length (zero days in this case)
‘=’ and > indicate if the lag must be equal to the specified lag length or may exceed it (that essentially means “strong” and “rubber” dependency)
You don’t need to be as verbose if you only need a strong Finish-Start dependency with zero lag. Just use a task number instead.