What is the difference between 是 and 在? I don't really understand the difference. Someone said zai is location, but it is used also in the sentence "Mum and Dad are drinking tea."
是 is used when a piece of information always holds true for the subject.

我是美国人。
I am an American.

Even though the person’s nationality might change, at the time of speaking, he wants to say that his current nationality is American. Note that he also describes himself as “American”, which is a noun. Hence, A 是 B means A is B (noun).
在 is used to describe a continuous action.

爸爸妈妈在喝茶。
Dad and Mum are drinking tea.

It shows that Dad and Mum are doing the action of drinking tea. If you said 是, then “drinking tea” would become a noun that is synonymous to “Mum and Dad”, and that makes no sense. Here, A 在 B means A is B (continuous tense of an action).
A similar concept in Spanish is the difference between “ser” and “estar”. Here, the “ser” is 是, and the “estar” is 在.