A transaction hash/id is a unique string of characters that is given to every transaction that is verified and added to the blockchain. In many cases, a transaction hash is needed in order to locate funds. It commonly appears at the top of the page when viewing your transaction through a block explorer website, or can be found by copying the URL from that same page.
Depending on the type of digital currency that was sent, you can find the transaction hash/id by searching your wallet address on a block explorer. Below are some popular block explorers for different crypto currencies:
Ethereum (ETH): https://etherscan.io/
Bitcoin (BTC): https://www.blockchain.com/