Have you ever played with Legos? Imagine that you have a big box of Legos, and you want to build something really cool with them. You can't just take one Lego at a time and build something awesome. You need to know how the Legos fit together, and you need to know what each piece does.
In a way, the Linux kernel API is like a big box of Legos. The Linux kernel is like the big box, and the API is like the individual pieces. The API is a set of instructions that tell the kernel how to work with other parts of the computer. These instructions are like the Lego pieces: you can use them to build something really cool with the kernel.
When you program in Linux, you're really just using the API. You're selecting the Legos you want to use and putting them together to build something new. To use the API, you need to know what each instruction does and how it fits together with other instructions.
And just like Legos, you can't just use any instruction in any order. There are certain rules you need to follow. But if you can figure out how to use the API correctly, you can build some really cool things and make the kernel do whatever you want it to do.