Every transaction has an ID, the txid. It is the double SHA-256 hash of the serialized transaction, shown in reversed byte order in block explorers and wallets.
What it is used for
Inputs point to earlier outputs by txid and output index (an "outpoint"). Blocks commit to txids through the merkle root.
txid vs wtxid
Since SegWit, the txid leaves out the witness. That makes it impossible for third parties to change the txid by tweaking a signature. The wtxid includes the witness, and for transactions without witness data the two are identical.