<p>A fillet is like a rounded corner but on the inside of the corner.</p>

<p><a href="https://i.stack.imgur.com/XDTnd.png" rel="noreferrer"><img src="https://i.stack.imgur.com/XDTnd.png" alt="enter image description here"></a></p>

<p>Does it make a difference (structurally) to use fillets on a 3d printed part?</p>

If your part needs structural support, then the word is:
absolutely
. Fillets provide the added support when you need it. If your part has a meeting line which is sharp - 90° (or perpendicular), there is a natural
stress riser
in your design. This is a weak spot where a crack can form. If strength is needed and the fillet won't interfere with the design, it's definitely something you should include with your part.