The attitude of a boat in the water can largely be controlled by trimming the outdrive/propeller unit in relation to the boat's stern.
When the drive is trimmed up, the prop exerts more downward force, causing the bow to lift out of the water.
Conversely, when the drive is trimmed down, the force is exerted more in line with the hull, resulting in the bow riding lower in the water.
Depending on the trim angle, you can observe increased or decreased speed, fuel economy, comfort, and control at a specific prop RPM.
The trim gauge connects to a resistance-based sender located in the pivot of the outdrive.
This operates similarly to the sender in a gas tank, with the gauge measuring the sender's resistance from fully down/in to fully up/out.
This enables the operator to quickly and easily adjust for conditions and load.