The value of the littlest MTU in a given path.
Given the following path between two hosts A and B:
HOST A ----- 1500 ------ GATEWAY1 ------- 500 -------- HOST B
As you can see in the path between the host A and the host B there are two segments,
the first with MTU 1500, the second with MTU 500. The Path MTU is the minimum
value of all the MTU presents in the path, so it's 500 in this case.
The Path MTU is importante because in order to avoid fragmentation modern operating
systems try to discover the Path MTU (using an algorithm called Path MTU discovery)
in order to send to a given destination only packets that will not get fragmented.