Kant believed in something he called the categorical imperative. A categorical imperative is a particular moral position that holds in all possible situations - an unshakable moral law, in other words. For imperatives like "do not kill," this seems reasonable, but for others, such as "do not lie," it gets a little hairier.
Imagine a scenario where a murderer comes to your door and asks if you've see your friend around. Moments before, your friend came to you telling you about the murderer, asking if they could hide at your house. Kant would say you're obligated not to lie, so your options are to either shut the door on the murderer (not a great idea) or give away your friend's hiding place (an even worse idea). You can see how a little white lie wouldn't hurt, and would in fact prevent harm from happening. If you were a sworn Kantian, it might play out badly for everyone involved.
To answer your question in light of that, Kantian ethics hold that certain moral standards are universal and impose a duty on all humans. Do not lie. Period.