Mono class in Java: what is, and when to use?

I have this following code:

import org.springframework.http.MediaType;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.BodyInserters;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

@Component
public class GreetingHandler 
    public Mono<ServerResponse> hello(ServerRequest request) {
        return ServerResponse.ok().contentType(MediaType.TEXT_PLAIN)
        .body(BodyInserters.fromValue("Hello Spring!"));
    }
}

I understand this code except what the class Mono does and what are its features. I did a lot of search but it didn't goes straight to the point: what is the class Mono and when to use it?

A Mono<T> is a specialized Publisher<T> that emits at most one item and then (optionally) terminates with an onComplete signal or an onError signal.
It offers only a subset of the operators that are available for a Flux, and some operators (notably those that combine the Mono with another Publisher) switch to a Flux. For example, Mono#concatWith(Publisher) returns a Flux while Mono#then(Mono) returns another Mono.
Note that you can use a Mono to represent no-value asynchronous processes that only have the concept of completion (similar to a Runnable). To create one, you can use an empty Mono<Void>.

Mono and Flux are both reactive streams. They differ in what they express. A Mono is a stream of 0 to 1 element, whereas a Flux is a stream of 0 to N elements.

This difference in the semantics of these two streams is very useful, as for example making a request to an Http server expects to receive 0 or 1 response, it would be inappropriate to use a Flux in this case. On the opposite, computing the result of a mathematical function on an interval expects one result per number in the interval. In this other case, using a Flux is appropriate.

How to use it:

Mono.just("Hello World !").subscribe(
  successValue -> System.out.println(successValue),
  error -> System.err.println(error.getMessage()),
  () -> System.out.println("Mono consumed.")
);
// This will display in the console :
// Hello World !
// Mono consumed.

// In case of error, it would have displayed : 
// **the error message**
// Mono consumed.

Flux.range(1, 5).subscribe(
  successValue -> System.out.println(successValue),
  error -> System.err.println(error.getMessage()),
  () -> System.out.println("Flux consumed.")
);
// This will display in the console :
// 1
// 2
// 3
// 4
// 5
// Flux consumed.

// Now imagine that when manipulating the values in the Flux, an exception
// is thrown for the value 4. 
// The result in the console would be :
// An error as occurred
// 1
// 2
// 3
//
// As you can notice, the "Flux consumed." doesn't display because the Flux
// hasn't been fully consumed. This is because the stream stop handling future values
// if an error occurs. Also, the error is handled before the successful values.

sources: Reactor Java #1 - How to create Mono and Flux?, Mono, an Asynchronous 0-1 Result

it might be helpful: Mono doc