A transaction ID is a unique string of characters that acts as a reference number for a particular transaction on the blockchain. You may hear this referred to as a transaction hash, owing to the way this unique string of characters is created by a cryptographic hash function.
Every time you perform an action - such as buying, selling, bidding, or accepting a bid on the blockchain - a transaction ID will be created. By navigating to the "Activities" tab of your profile you can find the transaction ID for your particular transaction in the "Transaction ID" column. Clicking this string will open up your transaction details in a blockchain explorer website, which will allow you to access more detailed information about the transaction.
You can find a list of transactions associated with your wallet address by using a blockchain explorer. Below are some common blockchain explorers for Ethereum and Solana.
When you navigate to one of the sites above, you can enter your wallet address into the main search bar. This will return every transaction ever made by your wallet address and is a very useful source of information.