In astrological tradition, the Mercury governs thinking, speech, and how information is processed, while Cancer is a water sign read as nurturing, protective, sensitive, and tied to home.
So Mercury in Cancer is often read as a placement where communication style and the rhythm of the mind takes on a nurturing, protective quality. Tradition holds that someone with this placement tends to gathers and shares ideas in a way that feels sensitive — coloured by the water-sign temperament of Cancer rather than by any single rule.
None of this is a prediction or a fixed trait. These are tendencies astrologers have long associated with Mercury in Cancer, and how they actually show up depends on the rest of the chart — the houses, the aspects, and the other placements that this guide cannot see.
A general guide to astrological tradition — tendencies, not predictions. It describes the placement in the abstract and is not about any specific person.