(From Sharepoint Help)
About Web Page Security Validation
Web Page Security Validation enhances security by imposing a time limit on pages when the user is submitting information to the server. This feature ensures that the connection between the browser and the server is more secure, and that data is not altered on a user's behalf without his or her knowledge. When users take too long before submitting changes to the server, they receive a message informing them that they must go back to the previous page and retry the operation.
In most installations, a setting of 30 minutes is usually appropriate. If site users experience frequent time-outs because of long data transfer times, consider increasing the interval. However, you should not turn off Web Page Security Validation, as it helps to maintain the security of your server.