Consumer behaviour is a maximisation problem. It means making the most of our limited resources to maximise our utility. As consumers are insatiable, and utility functions grow with quantity, the only thing that limits our consumption is our own budget (assuming, of course, we are dealing with normal goods, not negative or harmful goods which consumption we want to limit).
A budget constraint (green line in the adjacent figure) provides the second half of the maximisation problem. We need to balance the utility we derive from consumption with the budget we have.
Supposing we have a choice of two goods, 1 and 2, then our restriction is as follows:
which simply means that our budget must be at least as much as the price of the two goods times their respective price.
This simply shows that our consumption is capped and that the more we spend on one good, the less we can on the other.
Video – Budget constraint: