Chip damage is a new gameplay mechanic introduced into Super Smash Bros. Clash. This is a new mechanic that has been in traditional fighting games for a very long time.
Chip damage is the damage a move inflicts when a Special move is blocked. Usually, this is extremely small. Some normal attacks, such as Bowser's Forward Smash do chip damage as well. Not every special in the game does chip damage however: Good examples are Fox and Falco's Blasters and Funky Kong's bananas when thrown separately.
Chip attacks can refer to a move that is extremely safe on block and thus can be used to inflict some quick, difficult-to-punish damage. A good example of a move mostly used for chipping is Ryu's Shinku Hadoken Clash Attack.