How does a program ask the kernel for something?

A program asks the kernel for something through a system call. A system call is the way a program requests a service from the operating system it runs on, such as reading a file, using the camera, or starting another program.

Why can't a program just do these things itself? Because the processor and the operating system keep programs in a box. A program is usually limited to its own address space, so it cannot touch other programs or the operating system, and it is usually prevented from directly manipulating hardware. Yet applications do need hardware, so the operating system offers system calls as well-defined, safe ways to ask.

Here is the trip. The program triggers a signal, often an interrupt. The interrupt puts the processor into an elevated privilege level and hands control to the kernel, which looks at the request and determines whether the calling program should be granted the service. If it is, the kernel runs its own instructions, which the program cannot see or change, then lowers the privilege level and returns control to the program.

Think of a bank counter. You never walk into the vault; you ask the teller, who checks you may have what you asked for, fetches it and slides it across the glass.

In daily life you never notice any of this. Programs usually reach system calls through a library that sits between them and the operating system, so a programmer just writes "open this file" and the library does the knocking.