Grabbing is a way of laying claim to a title that you do not already have a claim on. To grab a title, click the coat of arms of the character whose title you want to claim. This will bring up the actions pane in the left hand of the screen. Click grab title.
In order to grab a title you need to spend money and you will lose prestige, as you are manufacturing a false claim. Claiming kingdoms will cost more than duchys , which will in turn cost more than counties . Grabbing titles will also worsen your reputation .
Having a claim on a character allows you to declare war , if you are both Christian leaders.