Rainbow Six Extraction has introduced a lot of new elements to the Rainbow Six series, from fighting aliens to reworked operator abilities. One of the other new elements is the concept of MIA operators.
An operator that fails during a mission or is eliminated by Archaeans will be deemed MIA, or missing in action. This MIA operator won't be playable until they are rescued. Failed rescues will bring operators back with penalties that significantly impact gameplay, so it's worth learning how to properly rescue an MIA operator.
To rescue an MIA operator, you have to return to the level where you lost them. You can see where to rescue each operator on the operator select screen or on the level select menu. You'll see a red icon showing MIA operators here, indicating which hot-zone within the mission contains them.
Once you drop into the hot-zone where the fallen operator is located, you'll have to locate an Archaean Tree. The creepy tree will be accompanied by a beeping sound, letting you know you're in the right area.
You'll see that the Archaean Tree has three branches that emerge and stretch to a spot on a wall or other surface. These are called Anchor Points. Destroying these Anchor Points while they're glowing will stop pulses along the branches that slow down the rescue. You should also shoot the pulses while they travel, not letting them reach the Archaean Tree.
Go up to the tree and pull the operator. Only stop pulling the operator to shoot the pulses before they reach the tree. You should try to shoot the Anchor Points as well to temporarily stop the pulses. You'll need to repeat this process for a while — pulling the operator, stopping the pulses.
When the operator is pulled from the Archaean Tree, you need to carry the operator to an extraction point. When you get to the extraction point, put the fallen operator into a pod. From there, you can leave with them or go to the next hot-zone.
A successful rescue mission will return the operator to you with no penalties. They will have a temporary drop in their health but it will return as you use other operators on missions.
If you fail the rescue mission, however, the operator will come back to the roster with a 30% reduction on their XP and contribution to your REACT progression. They will also have a temporary reduction in health.