Blockchain explorers are websites that allow everyone to see all the details of every transaction that took place on the blockchain. Think of them like public registers for every crypto transaction. Each blockchain usually has its own blockchain explorer -- like this one for Elrond.
To use a blockchain explorer, find the search box and enter the transaction tx identifier: the string of numbers and letters that identifies each transaction or the address for a wallet.
Through a blockchain explorer, you can potentially track all the activity of any wallet and obtain the official proof that a transaction was made.
It is also useful to use a blockchain explorer to check the status of a transaction. When the status of the transaction goes from "Pending" to "Success" you know your transaction has been securely processed.