Users selection:

u - user: the owner of the file.
g - group: users who are members of the file's group.
o - others: users who are not the owner of the file or members of the group.
a - all: all three of the above, is the same as ugo.

Permissions:

r - read
w - write
x - execute

So, u+x => Execute (x) permission to Owner (u).