What does it mean if a situation has a conditions, or constraint?

It means that the variables that describe that situation can assume only some specific values, or range of values

Step-by-step explanation:

You can have several examples of constraints. For example, you may have a geometric constraint. Assume that you have a circle of radius 10, and you want to draw a smaller circle, concentric to the outer one. Then, you want to study the ratio between the areas of the two circles, which is

In this case, the variable r is bound to stay between 0 and 10, because it makes no sense to have a negative radius, and on the other hand it cannot be more than 10, because it has to be an inner circle.

In other problems, you may have physical constraints. For example, if you're dealing with a physics problem involving a spring, you'll use Hook's formula:

where k is a constant (depending on the spring) and x is the elongation of the spring. Even though is a line defined over all the real numbers, in our case we can only accept a subset of values for x: the spring cannot be compressed neither stretched indefinitely.