Example
Alice has $8 to buy chocolate (X) and strawberries (Y). So she can buy chocolate and strawberries, as long as she spends less than $8.
The price of a chocolate bar is $4. She finds strawberries for $2 each.
Her budget constraint is $$4X + 2Y \leq 8$$
A tiny bit of algebra gives $$Y \leq - 2X + 4$$
The budget constraint is the shaded area in the plot above.
Question
Alice has $258 to buy chocolate (X) and strawberries (Y). The price of a chocolate is $74 and the price of a strawberry is $6.
Can she afford 4 chocolate bars and 3 strawberries at the same time?
In total, 4 chocolates and 3 strawberries cost Alice
$$74 \times 4 + 6 \times 3 = 314$$
No! Alice cannot afford 4 chocolate bars and 3 strawberries.