What does it mean when u don't have your period you only spot?

Depends. If you miss your period or only spot, you could be pregnant or you could have a lighter than normal period. I recommend getting a pregnancy test. If that is negative, then there are many many reasons for spotting. Much depends on your age, if you are on contraception, and many other factors.
Many conditions. If you are on contraception then itr is okay to have light cycles. Otherwise see your gyne.