A motion for leave is a request to file something that is not automatically allowed under the law. Often, a motion for leave to file is used to request a time extension from the court. Once the court receives the motion, the judge may either grant or deny it.
Rules of procedure, which vary between courts, govern matters related to filing lawsuits. If a party must deviate from the rules in any way, that party must generally file a motion asking the court whether the deviation is acceptable. Sometimes, if a party does not request leave to break the procedural rules, the party may lose the lawsuit and have their claim dismissed or receive a judgment against them.