to refresh / brush up
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to brush up
To reactivate and renew something that one used to know or be able to do. This often refers to knowledge, skills, or memories.
Ich muss vor der Reise mein Französisch wieder etwas auffrischen.
I need to brush up my French a little before the trip.
'Auffrischen' (to refresh/brush up) is a separable verb. The prefix 'auf-' comes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Ich frische vor dem Urlaub meine Italienischkenntnisse auf.
I brush up my Italian skills before the vacation.
This meaning is almost always used in relation to knowledge, skills, or social contacts. Typical objects are 'Kenntnisse' (knowledge/skills), 'eine Sprache' (a language), 'Erinnerungen' (memories), or 'eine Freundschaft' (a friendship).
Wir haben uns nach langer Zeit getroffen, um unsere Freundschaft aufzufrischen.
We met after a long time to refresh our friendship.
The verb can be used reflexively: 'sich auffrischen' means to tidy up one's appearance again, e.g. to wash or comb one's hair briefly.
Nach der langen Zugfahrt wollte ich mich im Hotel erst einmal auffrischen.
After the long train ride, I first wanted to freshen up at the hotel.
In connection with the weather, 'auffrischen' (to freshen) has a very specific meaning: the wind gets stronger. People often say: 'Der Wind frischt auf.' (The wind is freshening.)
Am Abend soll der Wind stark auffrischen.
In the evening, the wind is expected to freshen strongly.
to repeat / retrieve
'wiederholen' and 'auffrischen' can both fit when talking about learning or knowledge. 'wiederholen' means to review learning material again so that you remember it better. This can also be shortly after learning it. 'auffrischen' is used when you used to know or be able to do something, but it is no longer fully active. Then you bring it back into your memory. Learners may confuse the words because both have to do with learning again. Practically speaking: for current learning material, you are more likely to say 'wiederholen'; for old knowledge, 'auffrischen' is more likely.
Nach vielen Jahren möchte Sofia ihr Französisch auffrischen.
After many years, Sofia wants to brush up her French.
Vor der Prüfung wiederholt Max die wichtigsten Regeln noch einmal.
Before the exam, Max reviews the most important rules once more.