CSGO coinflip sites are sites that are offering a possibility to gamble CSGO skins on a game called coinflip. Coinflip is a PVP (Player Versus Player) game in which two users play against each other. They are betting the same amount of money and the winner gets all. So you can win around 2x of your bet (it may be less, because sites take a rake, mostly around 5-10%).
CSGO sites can offer two types of coinflip. One type is the one where you have balance and can bet some amount of coins on coinflip. The other one is coinflip for skins, where you have virtual inventory on the website and bet skins directly. If you win in coinflip for virtual items, you will get skins instead of coins.
If you are betting on coinflip, your odds will not be always 50-50. If you are gambling on a CSGO coinflip that is operating with skins instead of coins, it's possible to deposit a bit less or more (usually up to 5% difference) than the person that created coinflip.
Many of these CSGO coinflip sites are offering bonuses for new users and active ones. On some of them, you can claim free coins without depositing and try to win something out of them. To claim a referral bonus, just use the referral code provided with the website.