what is MVC in salesforce?

This is a good link -
https://developer.salesforce.com/docs/atlas.en-us.216.0.pages.meta/pages/pages_intro.htm

Salesforce has created a comprehensive platform for building on-demand applications. Like other sophisticated application development platforms, the Lightning platform offers separate tools for defining:

(M) The structure of the data—that is, the data model

(V) The layouts that specify how that data should be displayed—that is, the user interface

(C) The rules that detail how that data can be manipulated—that is, the business logic

Splitting up application development tools based on whether they affect the data model, business logic, or user interface is also known as the Model-View-Controller (MVC) application development pattern—the Model is the data model, the View is the user interface, and the Controller is the business logic.

MODEL
The model is your database objects in Salesforce. The include the standard Salesforce objects like Leads, Contacts, Accounts, Opportunities etc but it also includes any custom objects you've created.

VIEW
The view represents the presentation of the data (i.e. the user interface).

Pages - While often just called "pages", what we are talking about is Visualforce pages. They are the building blocks of the user interface. Visualforce uses HTML to lay out the appearance of the application interface. Each page is referenced by a unique URL just like a regular webpage. The pages themselves also contain Visualforce Components which can be invoked by simple tags inside the page.

Components - these are both standard and custom Visualforce Components. Think of them like widgets that you can add to your pages. Once you write the code once, you can reuse it on multiple pages. Components are important because they allow for this reuse. Components can be styled with CSS.

CONTROLLER
Controllers are the building blocks of the actual application logic. The controllers are written in Apex code and they end up controlling and enforcing all the business logic.