When you make an API call, what you're really doing is writing a very short, very specific 'shopping list'. You're asking your computer to go and fetch you something from somewhere else - and the 'shop' is a datastore on someone else's computer which has been set up as an API server. You can see this in action if you use something like Google search to find some information. When you submit a search query, you're talking to Google's API, and it's fetching you data in the form of relevant search results.
For example: https://www.google.com/search?q=lego+millennium+falcon sends a request for links to other web content containing the keywords 'lego', 'millennium' and 'falcon'.
So, working with the shopping analogy, let's imagine you are in the middle of making dinner and realise you're out of some key ingredient - milk, for example. You need to send someone to the nearest shop while you carry on cooking. The only problem is, that person is very literal minded and needs precise instructions ...
How are you going to make sure they come back with the right thing?