Bird-Meertens formalism is like a special language that people use to tell computers what to do. In this language, people can describe things called functions, which are basically sets of instructions that tell the computer what to do with a certain set of input.
So let's say you want to create a function to add two numbers together. In the Bird-Meertens language, you would write something like this:
add x y = x + y
This line means that when the computer has two input values (called x and y), it should add them together and return the result.
The cool thing about the Bird-Meertens formalism is that it allows people to describe functions in a very clear, simple way. This makes it easier for people to write complex computer programs and for other people to read and understand them.
But be warned: Bird-Meertens formalism can get pretty complicated fast! So while it may sound like a language for five-year-olds, it's actually a very powerful tool for computer scientists and programmers.