In Crossfire, shooting is resolved by rolling different numbers of six-sided dice, and counting the number that come up five or six. Each die, therefore, has a one-in-three chance of scoring a "hit". If no die scores a hit, then the enemy is unaffected by the shooting. One hit is a "Pin" which stops the enemy advancing, two hits is a "Suppress" which makes the enemy dive for cover, and three hits or more is a "Kill".
Different numbers of dice are rolled depending on what weapon is being used, and what cover the target has. For example a machine gun rolls 4d6, and rifles roll 3d6. Each rolls one die less (3d6 and 2d6 respectively) when the target is in cover. Some types of cover, such as bunkers and pillboxes, do not just take off a die from the number rolled to hit, but also require sixes for hits, not fives and sixes. This is called "minus one pip per die".
Troops can form "firegroups" which all shoot at once at the same target. Three rifle stands might all roll 3d6 against a single target in the open. If the first rifle stand rolls a Pin (one hit), as does the second, and the third, then this is neither a Kill nor a Suppress, because the effects are not cumulative.
I think that's all you need to know.