Everything going ashore gets vacuumed and inspected before your first landing, usually during the Drake crossing, and boots are washed on the way off and back onto the ship at every site after that. You're looking for seeds, soil and organic matter caught in velcro, pockets, boot treads and camera bag seams. It's tedious and it's the single most important thing keeping introduced species out.
South Georgia is stricter than the Peninsula because of its rodent eradication programme, which is one of the largest ever attempted anywhere. Expect a more thorough check there.
Practical implication for packing: gear with lots of velcro, deep treads and mesh pockets takes longer to clear. Bringing kit that's already clean saves you time in the queue.