If an Operator is marked “Inactive” in your Rainbow Six Extraction Operator roster, it means that they are unavailable for selection because they are injured. An Operator is considered injured if their HP is below 40. In order to heal these inactive Operators, you have to play more incursions with other Operators, and earn as much XP as possible. Every 300 XP you earn heals Operators in your roster by 1 HP. So, for example, if an inactive Operator has 30 HP, you need to earn 3000 XP to restore them to operational status, and 21,000 XP to restore them to their maximum HP of 100.
The reason you end up with inactive Operators in your roster is that, unlike in most games, an Operator’s HP is not automatically fully restored at the end of each mission. You can’t even restore it during a mission. Any healing received during an incursion – whether it’s from Medkits, Doc’s Health Shots, Finka’s Adrenal Surge, or using Anabolic Accelerant – is merely a temporary health boost that offers additional protection from damage but does not affect the Operator’s base HP.
In-mission health actually works more like armor than health. It’s a lot like how injured athletes are sometimes given an injection to get them through the next big game, but afterward, they need a long rest. So yes, no matter how much you boost your Operator’s health during an incursion, if their base health has fallen below 100, then they will need healing to reach full health again. And the only way to heal is to earn more XP.