What does it mean when we say a muscle contracts involuntarily? describe one body function that is linked to an involuntary muscle contraction?

It means we do not consciously contract the muscles, i.e, when we desire. Further, it means that the muscles are controlled by the autonomic nervous system.
Example of involuntary muscle contraction is peristalsis. These are muscle movements in the gastrointestinal tract that assist food move along the oesophagus to the stomach.