What does steam validating mean?
From a game’s library page, select Manage > Properties. Select the Local files tab and click the Verify integrity of game files… button. Steam will verify the game’s files – this process may take several minutes.
Because Steam is, at its core, a DRM system. When Steam starts up, it checks to make sure that the Steam installation is up to date. On my system, this takes all of about one or two seconds, or a bit longer if there is an update to apply.
How to fix «STEAM validation rejected» error in CS:S?
Basically it checks the game files and if it finds anything wierd about them it re-downloads those files.