Since the blockchain lists all transactions since the very first, an explorer allows you to track past or current transactions.
Each transaction is linked to a transaction ID (Txid or hash). You can search on the blockchain explorer using this Txid. You will then get the information related to your transaction.
Bitcoin explorers :
Bitcoin explorers make it possible to search for information contained in the bitcoin blockchain. For example, here is the page displayed during a search from the transaction https://blockchain.info (bitcoin search explorer)
1 : Search explorer
2 : Transaction number (Txid ou hash)
3 : Adress from which funds were sent
4 : Adresse to which funds are sent
5 : Number of transactin confirmations
6 : Amount sent
7 : Size of transaction in bytes
8 : Date and time of the transaction
9 : Transaction fees used in satoshi/byte