"This is not common."

Translation: Ini tidak umum.

[StephenL1]

Can anyone explain the difference between umum and biasa?

In English, the word common has many meanings eg Frequently seen, shared, plain, etc.

How would you say the opposite of rare?

[Rick392366]

'umum' = public, general, common.
'biasa' = usual, regular, common.

These words are sometimes interchangeable, like in this sentence here :
https://forum.duolingo.com/comment/29804739

But they're not 100% interchangeable.
Examples:
toilet umum = public toilet.
'toilet biasa' doesn't work here, that would mean something like a plain/common toilet.

angkutan umum = public transportation.
angkutan biasa, doesn't work here for the same reason.

How would you say the opposite of rare?

I think I would use 'biasa', but it depends on the context.
'Saya jarang minum bir.' = I rarely drink beer.
'Saya biasanya minum teh.' = I usually drink tea.