Like radio buttons, form data for selected checkboxes are name / value attribute pairs. While the value attribute is optional, it's best practice to include it with any checkboxes or radio buttons on the page.
Add a value attribute to each checkbox. For convenience, set each checkbox's value attribute to the same value as its id attribute.
<fieldset>
<legend>Is your cat an indoor or outdoor cat?</legend>
<label><input id="indoor" type="radio" name="indoor-outdoor" value="indoor"> Indoor</label>
<label><input id="outdoor" type="radio" name="indoor-outdoor" value="outdoor"> Outdoor</label>
</fieldset>
<fieldset>
<legend>What's your cat's personality?</legend>
<input id="loving" type="checkbox" value="loving" name="personality"> <label for="loving">Loving</label>
<input id="lazy" type="checkbox" value="lazy" name="personality"> <label for="lazy">Lazy</label>
<input id="energetic" type="checkbox" value="energetic" name="personality"> <label for="energetic"> Energetic</label>
</fieldset>