You can choose whether questions on your Contact Forms are optional or compulsory.
By default, every question aside from Name is optional
If a field is compulsory, it means that unless someone answers that question while creating a Contact using that Contact Form, they will be unable to press save.
Watch the walkthrough
https://www.loom.com/share/c2ee879fdc4c4070bfae6429f8f5bdeb
How to make a question compulsory
Go to the Edit Contact Form page of your Contact Form. How to do that:
Select My Apps in top menu
Scroll across and select Contact Forms
Press the cog icon next to the Contact Form you want to edit
You arrive on the Edit Contact Form page
Tick the Mandatory tickbox shown below the name of the field
The Name field is always set as mandatory
Scroll to the bottom and press save