SegWit stands for Segregated Witness, where Segregated means separated, isolated, and Witness is the signed thing associated with a transaction. Thus, SegWit is the separation of certain transaction signature data from the transaction. segWit has been activated on the Bitcoin blockchain.
The main benefit of separating signature and transaction-related data is that it reduces the size of the data stored in a bitcoin block. This gives each block extra capacity to store more transactions. It also means that the network can process more transactions and that senders pay lower fees.
Another major benefit of SegWit addresses is that they are backwards compatible, so it is possible to interchange bitcoins from SegWit addresses with bitcoins from Legacy addresses.