A statute of limitations is a law that limits how long a person can wait to bring a legal action after a law is broken.
When the time limit specified by the statute of limitations has run out, or “tolled”, any future legal action is said to be "barred". For example, if the statute of limitations for a particular crime is 10 years, it is generally not possible to prosecute for that crime twelve years after it took place.
With respect to copyright law, the statute of limitations will describe how long after an act of infringement a rights-holder can wait to bring a suit. It will also limit the number of infringing acts for which the rights holder may seek damages. The length of the time limit varies between jurisdictions, and the exact nature of the limits varies as well.
For instance, in the United States, there is a three year statute of limitations for copyright violations, although this is a generic limit, not specific to copyright. This three year clock begins on the date of the most recent infringing act. However, some courts treat this as a “rolling” three years, meaning that a rights holder can only seek damages for acts within the three years prior to bringing suit. Other courts have held that even if the infringing began more than three years prior to the suit, if the infringement was ongoing, and at least one act was within the last three years, the rights holder may seek damages for all of the infringing acts.