"Academic probation" is the most common term colleges and universities use to indicate that a student is not proceeding academically as required by the institution. "Academic probation" often means that a student's grades and/or GPA are not high enough to continue in school if they stay the same. Someone can be placed on academic probation for a variety of reasons, although all will be academic in nature. Students may be on academic probation because of their cumulative GPA or because of their GPA in the classes required for their major, for example.
Being placed on academic probation is usually not the same as being asked to leave college, however. Students are given a probationary period -- often a semester -- to demonstrate that they can indeed make successful academic progress. Students may need to increase their GPA by a certain amount, pass all of their classes, or meet other requirements, as determined by their school.
If you are on academic probation, make sure to be perfectly clear on what you need to do in order to stay in school. The specifics of your probation, as well as how long your probationary period will last for, should be outlined in the notification you received from your school. And if you're not clear, ask as many people as possible until you find out the information you need.