The probe is attached to the bolt and pushes the pellets out of the magazine into the barrel.
These o-rings sit around this probe and ensures an airtight seal in the barrel when shooting.
It can wear out over the years, so a spare is handy!
If this seal is missing, your pellets will not come out of the barrel and you have a chance of double loading.

Tip: extend the lifetime of these o-rings with some periodic silicone grease/oil.