Wiki has a great textbook explanation of what is MOA -- minute of arc. See above.
Minute of arc equals approximately one inch wide at 100 yards. Just a coincident though, since inches and yards have nothing to do with minute of arc.
It equals approximately 1/2 inch at 50 yards.
It equals approximately 1/4 inch at 25 yards.
It equals approximately 1/8 inch at 12 1/2 yards.
Minute of arc is a measurement of an angle.
The closer you are, the narrower it is.
The further away your are, the wider it gets.
Same as with any other angle.
Salvo laid it out nicely for you. Now if you will read the Wiki article you will understand why it is.