Learn Hardcore Russian: Настрой яркость на планшете, иначе читать будет неприятно. - Adjust the brightness on the tablet, otherwise reading will be unpleasant.

Настрой is the informal singular imperative (ты-form) of the perfective verb настроить (to set/adjust/configure). In everyday speech it’s a natural command meaning Adjust it (and finish the adjustment).

What’s the difference between Настрой and Настраивай?

Настрой (perfective imperative) = make the adjustment (as a completed action), often sounding like do it now / get it done.
Настраивай (imperfective imperative) = be adjusting / adjust (process-focused, repeated, or ongoing).
In this context (one specific brightness change), Настрой is the most idiomatic.

How would I say this politely or to more than one person?

Use the plural/formal imperative:

Настройте яркость на планшете, иначе читать будет неприятно.
Настройте works for you (formal) or you all.

Why is it яркость (not some other form)? What case is it?