A cash game is a safer contest where you have about a 50% chance of winning.
50/50s, head-to-head matchups with individual DFS players, and double-up contests are all considered cash games.
Lineups entered in these types of contests are usually filled with more consistent players or players with a higher floor instead of "boom/bust" players. You'll also find a lot of "chalk" players in these lineups.
Tip: A "chalk" player is a player with a high ownership projection (often based on the player having a great matchup for that contest).