From Halopedia, the Halo wiki
A kill-to-death ratio, occasionally referred to as "K/D", is a player's kill to death spread ratio. It is a common statistic descriptor used in most multiplayer video games to inform the individual of one's performance in a game session.
A player's K/D is their number of kills divided by the number of times they died. If they are both the same then it will come out as one. If the player has more deaths than kills it will be a number between one and zero. If the player has more kills than deaths, then it can be any number above one. Bungie.net rounds K/Ds to the nearest hundredth.
"Kill-to-death spread" is not to be confused with "Kill-to-death ratio", as K/D Spread is a player's (or team's) total kills, minus their deaths. K/D Spread allows a quick comparasion of players after a match (or teams particully in the Multi Team playlist), while K/D Ratio provides a more accuate comparison over all the matches a player has played in. Having a good K/D Spread can be as simple as it being +1 or higher, however in unbalanced games with one pro or newbie, having a K/D Spread in the top half of the players. In the Playlist Action Sack, Tug of War is a game variant that depends on everyone on a team to have a good K/D Spread, as points are removed for dying as well as added for killing.
In Halo 5: Guardians, a KDA (kill-death-assist) ratio is also commonly used, which is similar to K/D Spread, but it is calculated as the following: (A ÷ 3) + K - D, where "A" represents assists (counts as 1/3 of a kill), "K" stands for kills, and "D" represents deaths. A player's overall KDA for any type of multiplayer is calculated as their total KDA for that type of multiplayer divided by the total number of games (e.g. a player with 104 kills, 87 deaths, and 58 assists across 11 Arena matches will have an overall Arena KDA of 3.3). KDA is calculated slightly differently in Warzone Firefight.
TrueSkill ranking system