Generally, nominative cases happen to subjects, while accusative cases happen to objects.
In grammar, a "subject" is the one that performs the action in a sentence. (In this case, "we" are the ones who perform drinking, "we" is the subject)
An "object" is what is "being acted upon" or "towards who/what the action goes". (The accusative case happens to what is being acted upon). In this case, the "tea" is what is being drunk, thus "tea" is the object.
In Turkish, the accusative case happens to "specific objects". The accusative case is identified by a suffix in the end of the word. The effect in English is adding the definite article "the".
- Biz çay içeriz - Here, "çay" is in its normal form, not an accusative case.
It means "we drink tea" (general tea)
- Biz çayı içeriz - Here "çay" has got the suffix ı being an accusative case
It means "we drink
thetea" (specific tea)