Learn Swedish: Han fuskar, men han är ganska smart. - He cheats, but he is quite smart.

It softens the adjective, making it slightly weaker than if you just said han är smart. Ganska means "quite", "fairly", or "rather". It sits right in the middle of the scale: it is stronger than lite (a little) but definitely weaker than väldigt (very).

Wait, doesn't Swedish have a rule where the verb has to come second? Why is it ... men han är ... instead of ... men är han ...?

You are thinking of the V2 rule! However, men (but) is a coordinating conjunction, which is a word that simply glues two independent sentences together. Words like och (and) and men do not count as the first position in the new clause. Therefore, the subject (han) takes the first spot in the second clause, followed by the verb (är).

Is smart the normal Swedish word for "smart", or is it just English slang?

It is completely normal in Swedish! It was borrowed from English a long time ago and works exactly like a regular Swedish adjective. Because han is an en-word subject, it is just smart, but if you were talking about an ett word, you would add a -t (ett smart barn), and for plurals, you would add an -a (smarta barn).

What group of verbs does fuskar belong to?