Welcome to the forums.
Firstly, and most importantly, I would avoid key resellers, especially G2A.
The idea is that people put up unused game keys for sale to unlock games on digital sales platforms such as Steam. It's easy to end up with extra games keys that you don't want. For example, unwanted gifts or if you bought a bundle but there are games in that bundle you already own, you could sell those keys on these sites to someone else that wants them. It's kind of like an eBay for game keys and digital software. The key reselling sites of course take a cut of that sale and/or host ads on the site that's how they make money.
The first problem is you can only take these resellers at their word that the keys are genuine and haven't been used, so it's always a risk. Second issue, and this is especially true with G2A, these sites have been known to sell games bought from the original retailer with stolen credit card info, turning their store fronts into a front for fraud. G2A has found itself in hot water over this a lot of the last 2-3 years.
Yes, keys on these sites can be very cheep, much cheaper than buying direct from Steam, Epic Game Store, GOG, PSN, XBox Live or whoever it happens to be, but I personally wouldn't be tempted by them.