A chip runner is an employee of a card room whose job is to sell chips to players while the players are seated at the poker tables. Typically, only larger card rooms employ chip runners.
They are called "runners" because frequently they must hurry to satisfy a player's loud call for "Chips!"; if a player is in need of chips it is often because the game is attempting to proceed around a player and they are having to "owe some chips to the pot", which can get confusing for both the player and the dealer.
Chip runners typically cannot buy chips from players: when players wish to cash out, they must go to the formal cashier's cage somewhere in the casino. Chip runners can, though, assist players who wish to cash out by allowing them to color up, taking their small-denomination chips and exchanging them for larger denominations.
Typically, players are expected to tip chip runners, though often they are only tipped when the player colors up. When coloring up, the player fills a rack with chips of some denomination (for example, a rack of a hundred $1 chips), lays an extra $1 chip on top of the full rack as a tip, and hands it to the chip runner, who then hands the player a chip of the proper denomination (in the example, handing the player a single $100 chip). Occasionally, players tip chip runners when they purchase chips (e.g. buying a full rack of dollar chips, and giving one to the chip runner as they finish their work).
Chip runners will often stack the chips being purchased in front of the player. They may also assist in coloring up, by helping the player place their chips in the rack(s). The primary measure of good service from a chip runner, however, is prompt service. Generally, when a player needs to buy chips or needs to cash out, they want to do so as quickly as possible.
To call a chip runner, you may either ask the dealer to signal for one (in some rooms, in fact, this is required, since they may have electronic signalling available) or you may call for a chip runner out loud by calling "Chips!".
Small card rooms may not have chip runners at all, and players may have to perform their own cash-to-chip conversions at the cashier's cage. In other small rooms, the floorman or brush may act as a chip runner.