What is an SDL renderer?

I'm starting with SDL2 and having some trouble trying to understand what an SDL_Renderer is.

What is it? What does it do? What's the difference between SDL_Renderer, SDL_Window, SDL_Surface and SDL_Texture and how they are related?

I had issues with this when trying to understand this introductory code:

#include <iostream>
#include <SDL2/SDL.h>


int main()
{
/* Starting SDL */
if (SDL_Init(SDL_INIT_EVERYTHING) != 0) {
std::cout << "SDL_Init Error: " << SDL_GetError() << std::endl;
return 1;
}


/* Create a Window */
SDL_Window *window = SDL_CreateWindow("Hello World!", 100, 100, 640, 480, SDL_WINDOW_SHOWN);
if (window == nullptr) {
std::cout << "SDL_CreateWindow Error: " << SDL_GetError() << std::endl;
return 1;
}


/* Create a Render */
SDL_Renderer *render = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
if (render == nullptr) {
std::cout << "SDL_CreateRenderer Error: " << SDL_GetError() << std::endl;
return 1;
}


/* Load bitmap image */
SDL_Surface *bmp = SDL_LoadBMP("./Tutorial/res/Lesson1/hello.bmp");
if (bmp == nullptr) {
std::cout << "SDL_LoadBMP Error: " << SDL_GetError() << std::endl;
return 1;
}


/* Upload surface to render, and then, free the surface */
SDL_Texture *texture = SDL_CreateTextureFromSurface(render, bmp);
SDL_FreeSurface(bmp);
if (texture == nullptr){
std::cout << "SDL_CreateTextureFromSurface Error: " << SDL_GetError() << std::endl;
return 1;
}


/* Draw the render on window */
SDL_RenderClear(render); // Fill render with color
SDL_RenderCopy(render, texture, NULL, NULL); // Copy the texture into render
SDL_RenderPresent(render); // Show render on window


/* Wait 2 seconds */
SDL_Delay(5000);


/* Free all objects*/
SDL_DestroyTexture(texture);
SDL_DestroyRenderer(render);
SDL_DestroyWindow(window);


/* Quit program */
SDL_Quit();
return 0;
}

I was using Twinklebear tutorial (suggested on SDL Wiki) and looked also on SDL Wiki Documentation and some books. But all of them assume that I know these definitions.