Hi Alinta,
One way to think about it is that you're taking a trip from your starting point to your ending point in a given amount of time. Displacement just tells you how far and in which direction you'd have to go to take this trip. Since you also want to take the trip in a certain amount of time, average velocity tells you how fast, and in which direction you have to go if you want to go in a straight line between those points in that amount of time. That's why it's given by the displacement vector divided by the trip time.
If your start and end points are in the same spot, then you don't have to move at all to take the trip (displacement = 0) and your speed is obviously zero since you're not moving at all (average velocity = 0).
And yes, you're right about units: displacement is a measure of length between two points, so it's measured in units of distance. Average velocity is a measure of speed if you traveled a straight line between those points in a given time, so it's measured in units of speed: distance/time.
By the way, you'll probably see another type of velocity very soon in your studies: instantaneous velocity (that's why I emphasized average above). This is given, for example, by the speedometer on your car and the direction you're driving at every instant of your trip. If you run an errand and end up back at home, your displacement and average velocity are both zero, but your instantaneous velocity was obviously non-zero for most of your trip.