In astrological tradition, the Mercury governs thinking, speech, and how information is processed, while Sagittarius is a fire sign read as adventurous, candid, philosophical, and freedom-loving.
So Mercury in Sagittarius is often read as a placement where communication style and the rhythm of the mind takes on a adventurous, candid quality. Tradition holds that someone with this placement tends to gathers and shares ideas in a way that feels philosophical — coloured by the fire-sign temperament of Sagittarius 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 Sagittarius, 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.