What is it mean when it's difficult to urinate?

Several. This may mean several things. However, it usually means a slow, weak stream, urinary hesitancy, or voiding frequently. One can also void throughout the night affecting one's sleep cycle.
Possible infection. Difficulty urinating is often a sign of infection. This is more common in women. If you are a man it could be related to your prostate. Also, some medications can cause this problem. Also sexually transmitted diseases can cause this. You need to have an exam including a urinalysis.