Why should I add '?' in the validation of reactive form?

Question

I am creating a reactive form in angular 13. When doing validation I am only able to make it work like this:

 <div class="form-group">
    <label for="firstname">First Name : </label>
    <input type="text" class="form-control" id="firstname" name="firstname" formControlName="firstname" placeholder="Enter First Name">
    <div *ngIf= "!firstname?.valid && (firstname?.dirty || firstname?.touched)"> 
    <div [hidden]= "!firstname?.errors?.['required']"> First Name is required {{!firstname?.errors?.['required']}}</div>
    <div [hidden]= "!firstname?.errors?.['minlength']">  Min Length is 10   {{!firstname?.errors?.['minlength']}} </div>
  </div>  

  </div>

To validate the firstname. Why should I add the

?

character after firstname and errors?

Noting that in the component I have a get method like that:

  get firstname() {
    return this.contactForm.get('firstname');
  }