The word "spare" has multiple meanings, depending on its context.
1) In a general sense, it can refer to something that is extra or unnecessary, such as having a spare tire in case one gets flat while driving.
2) It can also be used to describe someone who is not involved in an activity, but available to take part if needed - for example, "I'm just the coach, I don't play; my job is to make sure everyone else stays fit and healthy so they are always ready to play. My role is to ensure that there are no injuries or fatigue – I am the spare man."
3) In a more specific sense, "spare" can refer to an additional ball in bowling, or an extra player on a team roster who does not usually participate in games but may replace an injured or absent player.