You need to read the fare rules to be sure, but in general:
(*) Most cheap United tickets are not REFUNDABLE (you do not get money back to your form of payment if you cancel the trip).
(*) Most cheap United tickets are CHANGEABLE and do carry RESIDUAL VALUE (it is usually possible to cancel an itinerary then pay a change fee and book a new flight, with payment for the new flight coming with credit from the originally booked flight, subject to certain time limits that have the effect that you can't use the credit very far in the future).
The UA message here is completely true (your L fare is probably not refundable) but maybe confusing (you can probably pay a change fee and use the value of your ticket to book a new one). In the UA structure you don't have to pay the fee until you book the new trip (but you do have to pay the fee with new money). You gotta get the new ticket issued before the value of the old ticket expires.
United has, occasionally but very rarely, issued some super cheap fares with terms that make them, not only nonrefundable but also nonchangeable and with no residual value ("use it or lose it"). These kinds of fares are very rare; it's difficult to find a "use it or lose it" fare.