An altar is a religious symbol, so the meaning of a dream about an altar will depend on your feelings about religion. It can represent safety, guidance, guilt or even dishonesty.
Because sacrifices were made on altars in ancient times, an altar in a dream can represent a sacrifice. You may feel that you have made too much of a sacrifice or be worried that a choice you have made or are about to make will cause you to sacrifice too much in the future.
Making a sacrifice can be a positive thing, however. If you are happy in your altar dream, you could be thinking about a sacrifice you have made or that you are about to make and about how happy this sacrifice has made you or about how happy it will make you. For example, you may be thinking about sacrifices you have made for a child or sacrifices that you have made to ensure that you will have a better future. The altar can also signify a sacrifice that you have made for a cause that you believe in.
People who share the same religious views will congregate together in front of an altar, so an altar in a dream can also represent the sharing of similar viewpoints.