What does the "U" symbol mean in the gene table?

The “U” stands for “uncharacterized protein” and means that there was no gene symbol available in the NCBI database. Many of these would show up as LOC#######, with the numbers matching the Entrez ID value.

In many cases, we have some idea of what the gene is but the confidence is not high enough to assign a symbol. Reading the gene description can usually give some meaningful information.