A cover of 2.5 means two and a half dollars of bids chased every dollar of supply. Higher is stronger, but the level only means something relative to the recent average for that specific tenor, because bills routinely cover above 3 while 30-year bonds run closer to 2.3.
Cover is a blunt instrument. Bidders can pad the ratio with bids far away from the market that were never going to be filled. The auction-tail is the sharper signal because it measures price, not volume.
Example: the trailing six-auction average cover for the 7-year note is 2.58. Today's auction covers 2.31. That is a 0.27 miss and, combined with a 1.5 basis point tail, tells you the market had to be paid to absorb the paper.
Related: treasury-auction, auction-tail, when-issued, primary-dealer