In Visualforce, the 'View State' is a hidden form field that maintains the state of the page (and the controller's variables) across postbacks to the server. If the View State becomes too large, it can slow down page loads and eventually hit the Salesforce View State limit (170KB), causing the page to crash.

The transient keyword is used to declare instance variables in Apex controllers that should not be saved in the View State. When a variable is marked as transient, its value is discarded after the request finishes and is not transmitted back to the client. This effectively reduces the size of the View State. It is commonly used for data that is needed only for the duration of the current request (like a large list of records displayed in a read-only table) and can be easily requeried or recalculated if needed.