is, evaluation
The is built-in predicate is used in Prolog to force the evaluation of arithmetic expressions. If you just write something like X = 2 + 4, the result is to bind X to the unevaluated term 2 + 4, not to 6. Example:
     ?- X = 2 + 4.
     X = 2+4

If instead you write X is 2 + 4, Prolog arranges for the second argument, the arithmetic expression 2 + 4, to be evaluated (giving the result 6) before binding the result to X.

     ?- X is 2 + 4.

     X = 6

It is only and always the second argument that is evaluated. This can lead to some strange-looking bits of code, by mathematical standards. For example, mod is the remainder-after-division operator, so in Prolog, to test whether a number N is even, we write 0 is N mod 2, rather than the usual mathematical ordering: N mod 2 = 0.

What is the difference between N = 1 and N is 1? In final effect, nothing. However, with N is 1, Prolog is being asked to do an extra step to work out the value of 1 (which, not surprisingly, is 1). Arguably, it is better to use N = 1, since this does not call for an unnecessary evaluation.

The message is: use is only when you need to evaluate an arithmetic expression.

Actually, you don't need to use is to evaluate arithmetic expressions that are arguments to the arithmetic comparison operators >, >=, < =<, =:= (equal), and =\= (not equal), all of which automatically evaluate their arguments.

Note the syntax of is: either
<variable> is <expression>
or
<numeric constant> is <expression>
Thus things like X*Y is Z*W do not work, as X*Y is an expression, not a variable - use either

0 is X*Y - Z*W
or
X*Y =:= Z*W
instead.

A common mistake, for people used to procedural programming languages like C and Java, is to try to change the binding of a variable by using an goal like N is N + 1. This goal will never succeed, as it requires N to have the same value as N + 1, which is impossible. If you find yourself wanting to do something like this, you could look at the code for factorial in the article on tracing for inspiration.

Another common mistake is to try a goal involving is before the variable(s) on the right-hand side of the is has/have been instantiated. Prolog cannot evaluate an arithmetic expression if it doesn't know the values of variables in the expression. This is why the following example fails:

?- X is Y + 1, Y = 3.
ERROR: is/2: Arguments are not sufficiently instantiated
Compare this with:
?- Y = 3, X is Y + 1.
Y = 3,
X = 4.