So – you’re thinking of posting an elephant, and trying to work out the most optimal way of doing it?…
That was the analogy I used when a friend, I was in the Irish Coast Guard with, asked me what on earth an MTU was (reference).  For anyone else wondering, here was my answer
MTU stands for “Maximum Transmission Unit”, or the piece of data (packet) that can sent in a single network transaction.  If, during transmission, the packet is too big, then it is “fragmented” or broken into smaller pieces – to be re-assembled when it reaches the destination. Packet fragmentation can cause delays and overhead, to setting an appropriately sized MTU is important for end-to-end performance.
Think of it like sending someone an elephant through through the post … you can send the whole thing at once, but then get frustrated when it takes forever to get to the intended destination (if the postal service doesn’t give up en-route), or you can dice it up in advance and just send pieces that you know will fit through the letter box.
See – more efficient… right?