Exactly how anti-gravity works in the Traveller universe isn't entirely clear in the Mongoose rules, so I'm doing my own clarifications so that I can be consistent. When I say 'works' I'm talking about the practical affect it has on ships and vehicles, not the techno-babble behind it.
Ships, space stations and micro-gravity settlements will often use gravity plates to generate a local artificial gravity field. This allows people to move around as if under 1g of acceleration. This also has the added benefit of reducing inertial effects from ship acceleration. Generally, such fields can be configured to cover anything from 0.5g to 1.25g, though more powerful fields are possible.
The plates are horizontal, and generate a mostly constant gravity field up to 2m from the surface. Every 0.5m beyond that, the field drops in intensity by about half. This can cause strange behaviour of items thrown above a height of 2m. Overlapping multiple fields has no effect as long as they are all on the same vector.
In large settlements, larger fields can be generated, often between 3m and 5m in height.
Starship manouevre drives can produce reactionless thrust. These drives burn hot, due to energy dissipation, but the range of the heat is normally not more than a few metres so is rarely useful as a weapon. The energy is directed though, so main drives are normally at the rear of a ship facing backwards.
A side effect of the drive is to produce a bubble field that blocks gravity, that often forms and ellipsoid around the ship. This reduces the local gravity field by about 0.25g per thrust rating of the drive. This is generally enough to allow even a 1g thrust ship to take off from a 1g world.
The field can be temporarily boosted, up to about five times its normal strength (so nullifying up to 1.25g standard gravity for a 1g drive) for a short while. This is generally used during take off and landing, but consumes an hours worth of energy every minute, so usage is normally kept to a minimum.
This gravity nullification often requires the use of gravity plates to allow normal gravity within the ship itself, otherwise the crew and passengers would experience low or zero g.
These work in a similar way to a starship drive, but are configured differently since they are designed to spend most of their time inside a gravity well and atmosphere rather than outside of one.
The gravity field nullification is configured to be up to 1.25g. Acceleration is much reduced, but power consumption is considerably higher - though much less than when a starship is operating in boost mode.