In law, evidence tampering refers to any attempt made to modify a piece of evidence used in a court case, usually for the purpose of tipping the scales in favor of one of the parties involved. Evidence tampering can be very simple, such as forging a signature on a document to make it look like a deal took place when it didn't - but it can also be a very complex and involved practice, for example breaking into a secured computer database and altering its information, as well as erasing security logs to prevent authorities from proving that the database has been tampered with.
Evidence tampering is a very serious offense, and some jurisdictions treat it even more seriously than false testimony (it should also be noted that in some rare cases, both are treated as the same act, though this only applies to some select few jurisdictions around the world).
Evidence tampering can occur at different stages in the course of a legal case. It can be committed directly after the crime itself, while police are still investigating the crime scene and collecting evidence - and most criminals prefer to do it this way whenever possible, as it minimizes the chances of proving evidence has been tampered with. It can also be done during the legal proceedings, by forging documents before presenting them to court, or even arranging to break into the police's evidence locker to steal/alter something - though the latter case is very rare due to the extremely high risk attached to it (as in that case, the charges pressed against the person if they are discovered will include breaking and entering in a police precinct apart from evidence tampering).