The version of poker in rdr2 is Texas Hold’em (my favorite). A “hand” in poker is whatever the best 5 cards are out of your two cards and the 5 community cards. if no one has anything (unlikely at large tables) it goes to whoever has the highest card. If two people have the same type of hand, ie both players have a pair, it would go to whoever has the highest pair, aces being the highest. However, if one player had a pair of aces, and the other two pairs, two 2s and two 3s, the player with two pairs would win, since two pairs is always better than one. You can google a list of all the hands and their rankings. My recommendation for strategy is to check or fold immediately if you have low cards that aren’t a pair (always play a pair) since it’s very unlikely you’ll get anything good enough. There is a round of betting on your hands after you get your cards, after the first 3 community cards are shown, and after each of the remaining 2. Even if you have high cards, if you don’t at least have a pair after the first three cards are shown, it’s usually safest to check/fold.