What is APP_BASE_HREF in Angular 10 ?

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import {APP_BASE_HREF} from '@angular/common';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    AppRoutingModule
  ],
  providers: [ {provide: APP_BASE_HREF, useValue: '/gfgapp'} ],
  bootstrap: [AppComponent]
})
export class AppModule { }

import { Component, Inject } from '@angular/core';
import {APP_BASE_HREF} from '@angular/common';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent {
  title = 'demo1';
  constructor(@Inject(APP_BASE_HREF) private baseHref:string) {
    var a = this.baseHref;
    console.log(a, " is base HREF")
  }
  
}