Pork barrel refers to money set aside in a bill for a legislator's home state. It's usually considered a form of kissing up to the home voters.
Originally, a pork barrel was an actual barrel full of pork, and it later meant any source of money. These days, it usually refers to something legislators do in Washington to please their constituents back home. Parts of a bill that set aside money for projects back home are called pork barrel. This is considered negative because the legislator is perceived as setting aside the money only to win votes back home, not for the greater good of the country.