LEI stands for Legal Entity Identifier and is an international identifier for legal entities, similar to an ID number. The goal is to enable international regulatory authorities to monitor transactions of individual legal entities, thereby improving oversight of systemic risk and facilitating investigations into securities trading misconduct. More detailed information can be found on the ESMA website.
As of September 1, 2021, all legal entities intending to trade financial instruments on organized trading venues must have a LEI identifier.