This is going back a ways, but basically the quadratic residue can be considered a remainder, or what's left over.
It's from an abstract form of modular integer arithmetic. the closest common examples I can think of to explain the concept are numerical schemes like binary, hexadecimal, octadecimal bit math, or something more familiar to us is 12 hour clocks.
These are all schemes using something other than base 10 (normal math is base 10, i.e. 1,2,3,4,5,6,7,8,9,10,11(1x10 + 1), 12(1x10 + 2), and so on. After you reach 10, the numbers repeat. Binary is base 2, Hexadecimal is base 6, etc.
For instance let's look at the clock which is base 12.
This can most easily be understood by saying it's 9 o'clock and you add 6 hours. Logically, you'd think it should be 15 o'clock...no, it's 3 o'clock because we hit 12, and then we repeat. 3 would be the residue it we were talking about "linear residue base (moduli, or mod) 12"
Now, lets get back to the quadratic part. Quadratic is a polynomial equation to the 2nd order. f(x) = mx^2+b
The independent variable is to the second power, or squared (squared = quad)
When referring to quadratic residue, in a modular system, we're in a realm of remainders of squares (like the clock, but non-linear, it's now squared), so instead of counting 1,2,3,4,5..., if we use [mod 2] it would be 2,4,6,8..., if we use [mod 3] we have 3,6,9,12..., and so on.
The expression is as follows [q is quadratic residue]:
q = X^2 (mod n)
So, mod (or moduli) is the base system modifier.
Let's say x is 3 and I want to evaluate it to mod 2
3^2 = 9; and if I use mod 2 (2,4,6,8) I can express this as 4x2 + 1
q=1 because the +1 from above is the residue or remainder after I count by 2's
Another example:
4^2 (mod 7)
4 squared is 16.
If I count by 7's....I go 7,14, 21...etc.
q=2 because I would express this as 2x7 + 2 to get 16.
Hope that makes sense.