CLSAG (Concise Linkable Spontaneous Anonymous Group) signature are the new way Monero builds ring signatures, which replaces MLSAG (multilayered Linkable Spontaneous Anonymous Group Signature). CLSAG will go live and mandatory with the Monero network upgrade of October 2020. CLSAG maintains the same functionalities as MLSAG, but with the advantage of considerable size savings for transactions and improvements in verification speed. This new technology was developed by contributors of the Monero Research Lab and audited by external researchers.
With CLSAG, users see a 20% improvement in signature verification, and at least a 10% overall improvement for typical transactions. For example, a typical Monero transaction (2 inputs and 2 outputs) which usually weighs 2.5kB, takes only 1.9kB of blockchain space with CLSAG, a ~25% improvement.