Each college has its own academic calendar, and it pays (sometimes literally) to familiarize yourself with that calendar.  

• The add deadline is the last day to register for a class. It's a good idea to register well before the add deadline, since popular classes often fill up early. But you can't register for any classes at all after the add deadline.  
• If you want to withdraw from a class, it's important to do it before the drop deadline. Sure, you can always drop out of a class later - but you won't be able to get your tuition refunded.  Dropping a class after the deadline will also affect your GPA and your transcript. Depending on your school and how long after the deadline, that class will show up as a W, a WU, or even an F on your transcript if you don't withdraw before the deadline. 

If you're not sure when the drop/add deadlines are at your college, make sure to check the academic calendar posted on your college's website. It's a good idea to talk with your academic advisor, mentor, or other trusted adult before changing your schedule though, since dropping a class could affect your progress towards graduation, or even your financial aid.