A 50/50 contest is a contest in which the top half of all entrants get paid and the bottom half lose their entry fee. These contests are considered safer based on odds of winning. All winning entries will be paid the same amount.
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).