Usually, you'd only have one GPU working on a display. However many laptops come with the ability to leverage two GPUs on one display. On such setups, these typically involve the iGPU, which is weaker but less power hungry, and the dGPU, which is more powerful but comes at an additional power cost.
Hybrid mode is when the iGPU is doing the typical GUI stuff and the dGPU kicks in for more GPU intense stuff like games.