Light has different prodigious properties. One of them is that light takes the time which is extremum. It means that the time should be either minimum or maximum.
One example which will help you to imagine the core of concept is of block. Imagine a block and now drop that block in air one time and then in water other time from same height. Now, you can imagine the timing in which the block will touch the ground.
Absolutely, the block will touch ground fatser in air than in water.
Similarly, we can think in case of light. As light will simply pass in air(vaccum) but will show reflection and refraction in water due to which its time changes. This change in time is either maximum or minimum.(Except normal incidence)
And the path it will follow in air is nothing but the geometrical path and the path it will travel in water is the optical path.
We can say simply, the actual path without any liquid (refractive index) is the geometrical path and when the light travels through any medium (liquid) then its path is said to be optical path.