A constraint is a condition that we must solve in order to satisy an x situation:
For example, if we have a total money of $18 for buying a drink or sometype of food, then we need to formulate a budget constraint to be able to satisfy our needs.
Like for example if the price of a beer is $2 and the food is $3 and we have an income of 18, then the spending on beer would be 2b and the spending of food would be 3b, so, total spending=2b +3b.
Therefore, the budget constraint would be equal to 2b +3b=18
So we would have a constraint with $18 of buying x amount of beer and x amount of food drinks.