Your transmission is similar to the gears on a bike. When the gears are perfectly aligned, your vehicle will run seamlessly. However, if the gears are misaligned, then driving, and in particular changing gears, will become vastly more difficult. You will notice this especially when you are trying to start your vehicle.
If the gears are in too high of an alignment, your vehicle may not be able to generate enough force to get the transmission started and thusly the rest of your car. By the same token, if your transmission is in too low of a car, your vehicle will not be able to achieve any of the top end speeds you need in order to safely commute on the roads. If these problems sound familiar, you need transmission shop repair.