Grenades are explosive, this means that they cause damage in a large radius from their point of origin. This can be extremely detrimental to yourself and those around you if you accidentally prime a grenade and it detonates too closely.
Do not use grenades unless you're extremely confident in your application of it, because it will more often then not do more damage to yourself and your fellow PFCs than the enemy. Grenades are also fairly useless in terms of risk vs reward because the risk is massive and the reward is usually subpar. If you do end up having to use a throwable explosive, inform your fellow marines that you are about to toss one, and make sure there are no friendlies within the blast zone.