Something that is sour has a sharp, unpleasant taste like the taste of a lemon.

Sour milk is milk that has an unpleasant taste because it is no longer fresh.

Someone who is sour is bad-tempered and unfriendly.

If a situation or relationship turns sour or goes sour, it stops being enjoyable or satisfactory.

If a friendship, situation, or attitude sours or if something sours it, it becomes less friendly, enjoyable, or hopeful.

If you refer to someone's attitude as sour grapes, you mean that they say that something is worthless or undesirable because they want it themselves but cannot have it.