The word level has several meanings:

1. A particular height or position compared to something else. The second level of a building is one floor above ground level. When floodwater reaches dangerous levels, it means the water has risen high enough to cause problems. In video games, each level presents new challenges as you progress through the game.
2. Flat and even, with no part higher than another. A level surface makes it easy to roll a ball in a straight line. Carpenters use a tool called a level to check whether a shelf is perfectly horizontal. When you level a playing field, you make it flat so the game is fair for both sides.
3. An amount, degree, or intensity of something. Your energy level might be low before breakfast but high after eating. Teachers assess your reading level to understand what books match your skills. Scientists measure pollution levels in the air and water.
4. To make equal or destroy completely. In sports, to level the score means to tie the game. In another sense, a tornado might level a building, knocking it completely flat.

The phrase on the level means honest and straightforward: “Is he on the level about helping us, or is this a trick?”