The default BlocklyLambda program you see below defines the (anonymous) function "plus one" (x is mapped to x +1) and applies it to the number 2.

"Generate JavaScript" compiles the program to JavaScript (which is a programming language that runs in your browser) and "Run JavaScript" executes it.

You can remove a program by dragging it out of the workspace. You can write your own program by dragging blocks from the toolbox to the workspace.

You can save your program to a file and load one from a file.

Lambda calculus at Wikipedia.