Hard to understand without mastering pure functions, but ok.
Partial Application is a function where some or all of the arguments are packed inside, ready to go and it's just waiting for a few more before the main function is invoked. They're like functions that have default arguments, but are pure functions with a fixed amount of parameters.
The function below pings google.com:
const pingGoogle = () => fetch('http://google.com'). If it resolves, Google is there and your computer can talk to the internets. If it throws an error, your computer is probably having wireless trouble.