It means you have received a share i.e you helped contribute to finding a block of MUE
1 block of MUE is worth 40 MUE, so you will be rewarded a % of that 40 MUE depending on how much hashpower was on that pool at the time you helped find the block.
e.g
Lets say you are contributing 100kh to the pool
The pool overall is 1000kh
that means you are 10% of the pool hashrate so you will receive about 10% of the block i.e 4 MUE out of 40

I'm no expert, but hopefully that makes sense