See below. Incarcerated hernia means it is not reducible (unable to push it back). Strangulation happens when blood supply to herniated organs is cut off. It is a very serious problem and can lead to gangrene within hours (needs emergent surgical intervention). When a hernia is strangulated the pain increases and gets worse over time. At late stages there might be redness of the skin overlying the hernia.
Strangulated hernia. A hernia is called strangulated if the contents are squeeze so tight that the blood supply is compromised. This causes severe pain, and usually swelling, redness and it will feel warm.