Think of it like this. Forget that the PUF is a physical device and just consider the "function" part. A function is a piece of math that maps input to output.

Think of it like this. Forget that the PUF is a physical device and just consider the "function" part. A function is a piece of math that maps input to output. For example, f(x) = 2x. This function is easy to compute and easy to predict. A hash function is easy to compute but hard to predict. A function doesn't have to rely on any kind of arithmetic, you can have a random function that simply maps all inputs to random outputs, or something silly like a function that hands its input to a two-year old, who then points out a star in the sky, then an astronomer takes a picture and gives that as a return to the function. A Physically Unclonable Function is a function baked into hardware in a specific way that, like hash functions, allows computation but resists prediction. The article describes in a general sense how this is accomplished, by making the output of the function rely on impossible-to-replicate physical features of the circuit. Even though the actual output is unpredictable, you can still tell it's from the PUF due to purposefully-added design constraints.