If you or your companions fall in battle, you (or they) will sustain permanent debuff injuries.
Right click on in your inventory, a radial menu should appear, mouse over all the options, the one on the left should be the 'use' option. Alternatively, you can move the injury kit into your quickbar. Then you can use it with a single left click.
As long as you have access to your camp, you do not need to use injury kits. Entering your camp heals you of all injuries.