Somebody bid more than you. That is almost always the whole answer, and it is worth saying first because the alternatives people reach for are not what happened.
Why it feels like something else
Your screen was behind. The price you were looking at was true a moment ago. On a slow connection the gap is wider, so the bid that beat you may have landed before you tapped. How fast does my internet need to be?
The clock went back out. A bid in the last ten seconds extends it, so the "final" moment you were watching was not final. That works for you as often as against you. The last ten seconds
Somebody had a maximum set. Their ceiling was bidding for them, one step at a time, without them being at the screen at all. You were not outpaced; you were outbid by a number decided earlier.
What did not happen
Nobody jumped the queue. Every bid goes through one database function that takes them one at a time, so there is a genuine first and second. What if two people bid at the exact same moment?
The seller did not bid. The app refuses a bid from the seller on their own item. Can a seller bid on their own items?
The clock was not stopped early. Nothing lets a seller end an item or hold it open.
The fix
Set a maximum. Enter the most you will pay and let it bid for you in steps. It beats tapping, it beats lag, and it beats the version of you that is ten seconds into a contest.
A maximum only goes up — it has already bid on your behalf, so it cannot be lowered — and it is not available on sudden-death items.
Tap the amount the app offers rather than typing one you read a second ago. The offered figure is the next valid bid; a number you remember may already be stale.
Decide the ceiling before the item comes up. There is no natural stopping point in an extending auction. How to not get carried away
If you lost by one step
You are the under-bidder, and that is a real position: if the winner does not pay, the item is offered to you at your own last bid — and letting that offer pass costs you nothing at all. What is an under-bidder?
So losing by one step is a good place to be. Losing by five is not.
If it happens every time
You are bidding below your real ceiling and then regretting it in the last ten seconds. Set the number first, as a maximum, and stop watching the clock.
More: what is sniping, the last ten seconds, why is my bid not the highest.