A straight-line PPF shows that the opportunity cost (what you give up) of making one good instead of another stays the same, no matter how much you make. This happens when the resources you use to make both goods are equally good at making each one. For example, if a factory can switch easily between making shoes and making hats, and for every pair of shoes they make, they have to give up the same number of hats, the PPF would be a straight line.
In real life, this is rare because most resources are better suited for making one good over another, but a straight-line PPF is used for simplicity.