fit
To be in good physical condition; athletic and capable.
Er geht dreimal die Woche ins Fitnessstudio, um fit zu bleiben.
He goes to the gym three times a week to stay fit.

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To be in good physical condition; athletic and capable.
Er geht dreimal die Woche ins Fitnessstudio, um fit zu bleiben.
He goes to the gym three times a week to stay fit.

'Fit' is usually used as a predicative adjective with verbs such as 'sein' (to be), 'werden' (to become), or 'bleiben' (to stay). It describes a person's condition.
Ich fühle mich heute nicht besonders fit.
I don't feel especially fit today.
The reflexive phrase 'sich fit halten' means to actively do something for one's own physical or mental health.
Sie hält sich mit Yoga und Schwimmen fit.
She keeps herself fit with yoga and swimming.
To express that someone is competent in a particular subject area, you use the fixed structure 'fit in' followed by a noun in the dative.
Mein Kollege ist richtig fit in Excel.
My colleague is really good at Excel.
Whether physical, mental, or subject-related fitness is meant can be recognized from the context. Words like 'Sport' (sports) point to the body, 'Prüfung' (exam) to the mind, and the preposition 'in' to a competence.
Für diesen Marathon musst du körperlich topfit sein.
For this marathon, you have to be in top physical shape.
ready
'bereit' and 'fit' can both express that someone can do something or is ready to get started. The difference is in the focus. 'bereit' means that someone is prepared and willing to do something. 'fit' describes more of a state: physically healthy, rested, mentally alert, or well prepared in a particular area. So you can be bereit even if you are not especially fit, for example when you are tired before an exam. And you can be fit but not yet bereit, because you do not want to start.

Julia fühlt sich heute fit genug, um nach der Arbeit noch zum Sport zu gehen.
Julia feels fit enough today to go exercise after work.
Nach einer kurzen Pause ist das Team bereit für die nächste Präsentation.
After a short break, the team is ready for the next presentation.