In HTML5, you can provide a required attribute to ensure a certain field has been filled out before a form can be sent off.

<input id="tac" required name="tac" type="checkbox">
<label for="tac">I have read the terms and conditions</label>

You can go even further and define a pattern that the value of the field needs to comply with before it can be sent off. For example, the following only allows entries that are either upper or lowercase a to z letters.

<label for="pkey">Product key (A-Z only)</label>
<input id="pkey" required name="id" pattern="[a-z|A-Z]+">

You can read up more on MDN and see some more examples.

Whilst this is an excellent way to get the browser to provide feedback to your users, it is not a security measure. Any form that relies on this feature alone can be defeated by using the Developer Tools Console and using JavaScript to submit the form.

$('form').submit()

You can see it here in action:

In any case, you always need to check the validity of any incoming data on the server side and, if necessary sanitise the data.