The final drive ratio is the last bit of gearing between your transmission and the driven wheels. By changing it, you can affect the performance of your car, rather dramatically in some cases.
In general, a lower final drive ratio will lead to less torque at the wheels but a higher top speed. Meanwhile, a higher ratio will result in the opposite, i.e. more torque at the wheels but a lower top speed. Remember, this is done without any change to the power and torque of the engine.
Since torque is what helps you accelerate, a higher final drive ratio will give you better acceleration. The catch is that your engine also needs to work harder, i.e. produce more revs, for a given speed, so the higher your final drive ratio the more fuel you’ll be using.
Jason from Engineering Explained gives a good explanation of how it all works and lists the pros and cons of changing your final drive ratio.
It’s also recommended that you watch his previous video on how car gears work if you’re unsure about how gears can affect the level of torque reaching the wheels.