Suppression of evidence is when a judge forbids a particular piece of evidence from being used in a trial. A judge will do this in circumstances where he or she believes that the evidence was obtained illegally, or when he/she believes that the attorney did not reveal the evidence to the opposing counsel.
Attorneys are required to reveal their discoveries in terms of evidence to the opposing counsels before a trial begins. This is part of the "due process" allowed to accused persons in the Fifth Amendment. So, if a prosecutor hides evidence and then brings it up during the trial, the judge will usually suppress it.
Evidence is also sometimes obtained illegally, for example: a police officer enters a house without a warrant and obtains evidence from the house. The illegally obtained evidence could be suppressed in court.