The score of a post is the sum of votes on it.
When someonesomeone* clicks the up arrow on a post (upvotes it) that increases its score by one.
When someonesomeone** clicks the down arrow on a post (downvotes it) that decreases its score by one.
This post has a score of 98:
When you have 1000 reputation points, you gain the ability to see the score breakdown by clicking on the score:
* this applies when the person voting has at least 15 reputation; otherwise the vote is anonymous***.
** this applied when the person voting has at least 125 reputation; otherwise the vote is anonymous***.
*** anonymous votes are recorded and information about them can be found, for example in the public data dump, but it does not affect the post score.