Explain use of zai4

Could someone explain the use of zai4 in the sentence

他问 小李 在 吃什么 and also in the following sentence

小李 说他 在 吃苹果…

I’m having trouble understanding it.

It means that he is “in the process of eating”. It conveys the meaning of the continuous form in English. If the sentence were

他问 小李 吃什么

it could mean that he asked him what he wanted to eat.

With the use of “在”, or quite often “正在”, we know that the person is in the process of doing something, i.e. eating.

thank you