Its an indication that this PSU is compatible with the ATX12V V2.3 standard. Newer boards with newer type of components (more fans, SATA Drives, etc..) required that the newer power supplies provide more power. The standard has gone from the old days AT --> ATX --> ATXV12 --> ?
uATX is just a form factor. You need to look at the motherboard and case. Your motherboard is going to have requirements for power and connectors. Make sure that the PSU you are buying meets the requirements. Then, you have to make sure that the case you are using has space for the power supply.
Ok, i'd just recommend looking at the motherboard specs. there should be information regarding what types of power supplies it requires. The case should also have that info. And with that, look at the power supply dimensions and it should all fit nicely.