Often you'd want to execute a statement if a certain condition is met, and a different statement if the condition is not met. This is what else is for. else extends an if statement to execute a statement in case the expression in the if statement evaluates to false.
The else statement is only executed if the if expression evaluated to false
Example
if (a>b) {
echo "a is bigger than b";
} else {
echo "a is NOT bigger than b";
}