The Eagles must decide if they are going to exercise the option to keep Foles and his $20 million salary in the week following the Super Bowl. If the Eagles do not excercise the option, Foles will become a free-agent. If the Eagles do exercise the option, Foles has five days to decide if he wants to pay back $2 million and void the option, becoming a free-agent.
If Foles accepts the option and stays with Philadelphia, the $20 million salary becomes fully guaranteed on March 18th, five days after the 2019-20 NFL season officially begins.
Got all that? Andrew Brandt summarized the situation well for SI.com if you want to learn more.