What is a budget constraint?
A Budget Constraint is the limit to what one can afford.
To simplify this idea, it is usually depicted by a simple line showing the trade-off one faces between two goods.
For instance, maybe I can spend my limited budget on 100 cups of coffee, 200 cups of tea, or some trade-off in between.
The slope of this trade-off line depicts the rate of the trade-off between the two goods.