
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.

Mentally ready, rested, or able to handle a particular task.
Nach einer guten Nacht Schlaf fühle ich mich wieder fit für die Prüfung.
After a good night's sleep, I feel fit for the exam again.

good at something
Colloquial for being competent or knowledgeable in a particular area.
In Mathe bin ich nicht so fit, aber in Sprachen kenne ich mich gut aus.
I'm not that good at math, but I know my way around languages.
'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 shows that someone is willing to take on a task and prepared for it. Fit describes here whether the person is mentally rested and capable enough to perform. A tired person can still be bereit, but that does not necessarily mean they are fit.

Nach einer erholsamen Nacht ist Jana fit genug, um die Präsentation zu halten.
After a restful night, Jana is fit enough to give the presentation.
Obwohl sie müde ist, ist Jana bereit, die Präsentation zu halten.
Although she is tired, Jana is ready to give the presentation.