It's difficult to say without a further history but it looks like a boil. This is an infected hair follicle. They can be quite painful.
For treatment try putting warm compresses on it several times a day. Just soak a cloth in warm (not hot) water and press it on. Hold it there until the cloth cools down. Several times a day and within a week or so it should be gone. In between, make sure you keep it dry and clean. You can also apply some "triple antibiotic ointment" to it, which is available over the counter.
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