Sem um maior contexto eu diria que é a capacidade de pensar e reagir com rapidez frente uma situação ou algo que acontece subitamente
.Think on one's feet
1. To think and react quickly, especially in a situation where things are happening very fast.
- I'd never heard about the company before, so I had to think on my feet.
2. An ability to think on your feet is a definite advantage when you're doing live comedy shows.
Fig. To be able to speak and reason well while (standing and talking) in front of an audience, especially extemporaneously.
- She really thinks on her feet well.
- I am not able to think on my feet too well before a bunch of people.
Source: The Free Dictionary