Used to be the indicator was a different color (I do not like the new format--I wish the powers that be would stop trying to make webpages look like mobile apps). It means that at the present time the bidder is the high bidder on that listing (leading bidder on an active auction or winner on a completed one--not sure whether high bidder but Reserve Not Met counts as either--it won't say which).
It doesn't make much sense to make a bid that wasn't the highest, does it?
If there is a prior bid by someone else you won't know whether the amount you are bidding is the highest until after you place it and then eBay tells you whether you are now in the lead or whether you were immediately outbid by the leader's hidden maximum. eBay hides the full amount of any bid that exceeds the price calculation ("current bid" while the auction is running, which is what the price would be if there is no further activity), substituting the calculated price for the actual amount.
