Block Explorer is an online block chain browser which displays the contents of individual blocks and transactions and the transaction histories and balances of addresses.
Each object is displayed in human-readable form, as a web page, and is given a URL. By using hyperlinks, it allows users to switch from seeing one piece of data to a related one, with a single click. Clicking on the hash of an object, will move to the page that displays its data. This way for instance, you can switch from looking at a transaction, to looking at the previous transaction which gave this transaction its inputs. All block data is visible, in human-readable or machine-readable forms, and even some information that is not actually part of blocks.
A blockchain explorer is a browser for the blockchain, similar to how we have browsers like Mozilla or Google Chrome for internet web pages.