|assignment||a is given the value 4|
All of these except for modulus should be familiar to you from elementary school. Modulus (%) gives the remainder from doing integer division. So 13 % 4 means that you divide 13 by 4, this results in 3 with a remainder of 1. Thus 13 % 4 is 1.
You can also use the addition operator with strings. In this case it concatenates (joins) the two strings together. A common mistake is adding (or forgetting) a space between the two strings you are joining.
Often you want to add (or subtract, multiply or divide) a variable by some value and then assign the result to the original variable. The assignment operator can be combined with these operators to do this in one step.