In general, an alter is a physical place were sacrifices are made to gods.
In the Old Testament, altars were places were animals were sacrificed to God as offerings or in payment for one’s sins. These sacrifices needed to be repeated again and again because people sinned again and again.
Alternately, altars were sometimes constructed to commemorate an event, agreement, or promise. Here their purpose was more like a monument, used to remind people of what happened.