A transaction is a record of a cryptocurrency transfer from one address to another on the blockchain, all transactions are recorded in blocks.
A transaction ID (txid) is a string of letters and numbers (the hash) of a transaction. Each transaction has a unique id for tracking transactions on the blockchain.
When the transaction ID is signed, it ensures that if any part of the transaction changes, the transaction ID changes and the signature becomes invalid.
A block explorer is software that allows anyone to easily browse and query on the blockchain using the transaction ID.