Learn Hardcore Latin: Cerebrum in capite est.

Cerebrum is nominative singular. In a simple sentence with est (is), the nominative noun is usually the subject, so cerebrum is the brain as the thing being talked about.

It is a neuter second-declension noun, and its nominative singular form ends in -um, which is very common for that pattern.

Why is it capite and not caput?

Because after in meaning in / inside / on, Latin normally uses the ablative case to show location.

The dictionary form is caput (head), but its ablative singular is capite.

So:

caput = head as a basic dictionary form
in capite = in the head

This noun belongs to the third declension, so its forms change less predictably than a second-declension noun.

Why does in take the ablative here?