Answer:
Suppose that the local school has $1,000 dollars. It can invest those $1,000 dollars to either buy 5 surveillance cameras each costing $200, or to buy 10 brand new textbooks for the library, each costing $100.
The school is facing a trade-off between buying surveillance cameras (guns), or textbooks (butter).
If the school only buys surveillance cameras, it will not have money to buy any textbooks, and viceversa.
The school board chooses the middle ground and buys 2 surveillance cameras for $400, and 6 textbooks for $600. Now, it does not have as many surveillance cameras as possible, or as many textbooks as possible, but it has a bit of both goods.
