"Rhaid i chi arafu."

Translation: You have to slow down.

[HerrArbo]

What is the purpose of the -u, how is it used generally?

[Julianbark6]

I don't think there's a "purpose" as such. It's just part of the word.

[RowenaJane]

Why not ' go slow'?

[matilim]

that's not what the verb "arafu" means

[silverlight513]

What's the difference between arafu and arafwch?

[Julianbark6]

Arafu is the basic form of the verb, equivalent to the Engish infinitive "to slow down" or present participle "slowing down".
Arafwch is the imperative plural, the instruction "Slow down!" (it's also the 2nd person plural future "You will slow down".