A blockchain explorer is a browser for the blockchain, similar to how we have browsers like Mozilla or Google Chrome for internet web pages.
A block explorer allows you to explore the whole entire blockchain of the platform you are using.
But it needs to be noted that you can’t use a block explorer for a blockchain it wasn’t meant for. For example, you can’t track Litecoin transactions with a Bitcoin block explorer. You’ll need a Litecoin block explorer for that.
For a deeper explanation on block explorers check out this link