There is no way in which we can guess what the writer means by these words. If they have their literal meaning, then a dictionary can help you. Otherwise, I recommend that you post the entire paragraph and explain what you think they mean in context. Then we may be able to help you usefully.

It's an introduction. Sometimes introductions aren't too helpful. It may be best just to skip it and read on. They will probably explain it more clearly. "Structure" probably means rules of grammar and the way a sentence is put together: subject, verb, object. Based on their paragraph, a "function" of a piece of language might mean "to answer a question," "to make a promise," or something else. They don't talk about "process." "Process" is probably the way the brain works to analyze and produce language. A possible example of a process might be "first hear sounds, then classify the sounds as phonemes, group the phonemes into words..."

It sounds like a textbook title. It doesn't really have a clear meaning. It also sounds vaguely as if the idea of the title is to "translate" some kind of technical term into simpler English, instead of saying "Textbook of Behavioral Psycholinguistics" or some such jargon. Without trying to define the words "function," "process," or "structure" precisely, perhaps an example of use might help. The function of a car engine is to provide power to move the car. The processes inside a car engine include compression, combustion, and exhaust. The structure of a car engine includes pistons, cylinders, a connecting rod, and a flywheel.

thank you teachers, this is the paragraph: Structure, Function, and Process Language is fundamentally an instrument of communication. People talk as a way of conveying ideas to others-—of getting them to grasp new facts, answer questions, register promises, and so forth. A language has not only a structure--what linguists try to capture with their rules--but also a function to which that structure is put. Psychology and language: an introduction to psycholinguistics. By HERBERT H. CLARK and EVE V. CLARK so my question is if you could give me a good explanation better than this :( ?