The user property provides an easy way to store some information related to a user. The concept is that information is stored as properties and subproperties, where each is just a keyword, so to store some information for the current user:

UserProperty::setPropertyForCurrentUser('windowsize', 'width', 900);

And to later retrieve the property:

$windowsize_width = UserProperty::getPropertyForCurrentUser('windowsize', 'width');