A high-output pickup sends a stronger signal to an amp, for a given amount of string movement/mass, compared to a lower-output pickup. This is a term that means different things depending on your perspective — describing a pickup as “high output” is relative, depending on what you are comparing or using as a reference. There is no set “standard” for what constitutes “high output.” However, the typical comparison is against a PAF-style pickup’s output. One way a pickup’s output may be increased by adding windings to its coil(s). This also results in the tone changing. Typically a higher output pickup is darker, with more midrange and low end. The tone and output can also be shaped depending on the type and strength of magnets used in the pickup.
Higher output means that the pickup will push a given amp into overdrive or distortion earlier. As a rule, dual-coil pickups have higher output than single-coil pickups, however these days some pickup manufacturers have specialty single-coil pickups for hard rock and metal with very high outputs. Active pickups are also often higher output than passive pickups.