If you find a bump on the side of your penis, it could be something serious.. Or it could be nothing at all... That's why it's always best to put your mind at ease by seeing a doctor.
Just like the skin on the rest of your body, the pores on the skin of your penis can become blocked by dirt, sweat, oil and all the other things that cause normal pimples and zits are.
Just like a pimple or zit on your face, the same can happen on your penis.
If this is the case, it might be nothing to worry about... A minor irritation that will go away in a short period of time.
On the other hand, it could be an infected wound or cut, some sort of boil, or even symptoms of an STD like herpes or syphilis.
So get yourself checked out by a medical professional at the earliest opportunity, and try not to worry to much in the meantime.