View State: As the name suggests, is an error of View State. Maximum view state size limit (135KB) exceeded is the error message pertaining to View State error and here follows a brief detailing of the issue.
View state holds the state/size of the visual force page that includes the components, field values and controller state. This is important in the light that salesforce provides only a standard size of 135kb for any individual page. If the size of a particular page exceeds 135kb, the page will throw a view state error.