Adrenaline Boost lets you buff a teammate so they ignore all damage for a short period. When the effect (indicated by a golden outline) wears off they'll receive all the damage they ignored, all at once. There is a long cooldown before you can re-use the ability, and you can't self-administer it.
This allows Medics to make other players invulnerable. Unfortunately the practicality of this Ability is limited due to the significantly short timespan of this effect. Another noticable downside is that when the effect wears off, the player is usually knocked down and has to get back up.
If timed correctly, it can be highly effective in ensuring that team mates succeed in planting or disarming HE charges or delivering data keys and samples.