A block explorer is a website specifically dedicated to indexing information on the blockchain ledger. Each network/blockchain has their own and through them you can view any transactions that occur on the appropriate network. This is very valuable when trying to review transactions and events that occur in your wallet.
Whether it is locating a token in your wallet after a trade, unable to view your farms in operation after providing liquidity, or being unsure of the effects of a transaction that you have signed – The block explorer will always tell you what events have occurred and be your source of truth. You can always query your wallet address to see a timeline of all actions taken.
