You will encounter many different characters or groups with whom you can build up your reputation. These are called factions. You will need to reach certain levels of reputation with certain factions to unlock new quests, areas, horses, and items. You will know that you need to improve your reputation to unlock a new quest when a character has a heart symbol above their head. Talk to the character to find out what to do to improve your reputation. You can see which factions you’ve encountered and your reputation progress on your Character Sheet.
Check your reputation under C > Reputation to see which faction you need to work on.