What is the question mark for next to TypeScript arg when its GraphQL arg indicates nullable true?

// image.entity.ts

import { Field, ObjectType } from '@nestjs/graphql';
import {
  Column,
  DeleteDateColumn,
  Entity,
  PrimaryGeneratedColumn,
} from 'typeorm';

@ObjectType()
@Entity()
export class ImageEntity {
  @PrimaryGeneratedColumn('uuid')
  @Field(() => String)
  id: string; 

  @Column({ default: false })
  @Field(() => Boolean, { defaultValue: false }) 
  isThumbnail: boolean; 
//isThumbnail?: boolean;  // it worked without question mark
}

//image.resolver.ts

import { Args, Mutation, Query, Resolver } from '@nestjs/graphql';
import { ImageEntity } from './entities/image.entity';
import { ImageService } from './image.service';

@Resolver()
export class ImageResolver {
  constructor(private readonly imageService: ImageService) {}

  @Mutation(() => ImageEntity)
  async createImage(
    @Args('imageUrl') imageUrl: string,
  //=============this part=================
    @Args('isThumbnail', { nullable: true }) isThumbnail?: boolean, // <--- that question mark I'm takin bout
    // @Args('isThumbnail', { nullable: true }) isThumbnail: boolean, // it work fine without the question mark
// what this thing for?  
  //========================================
  ) {
    return await this.imageService.create({ isThumbnail, imageUrl });
  }
}

Like on the code, it linked with TypeORM and GraphQL. And some of args (also related columns) are nullable.

I got it that {nullable : true} made both args and columns of GraphQL and TypeORM doable without the question mark. It worked fine, but on the nestjs doc, the question mark was right below the

@Field(()=>Some Type, {nullable: true}.

My question is that the question mark is just to indicate it is optional as typescript type?