Form Rules are just conditions which trigger actions. You do not need to be a programmer or systems administrator to understand how they work.

by the logic of IF THIS THEN THAT, you can create triggers like:

• If a field is empty/populated
• If a field value is equal/greater than/less than etc than other field
• If a field value is equal/greater than/less than etc than static value
• If a field value is equal/greater than/less than etc than URL param
• If there is/isnt a signature or attachment
• If User is viewing a form for longer than x Minutes
• others

Which can in turn trigger the following actions:

• Populate field with static value
• Populate field with URL value
• Populate field with other field’s value
• Hide/Show  Field, Section, Page, Related Object
• Make field Required/Not required
• Make field editable/read only
• Display a message

With the above logic, you can create simple as well as complex rules to add a layer of dynamism to your forms.