In Spades, what is the meaning of a high/low sum of bids?
A low/high sum of bids can mean anything.  The sum of 4,4,4,3 is the same as 1,1,1,12 (exaggeration), but those two mean completely different things.  Instead of looking at the sum of the bids, you get more information from looking at the individual bids.  

It's safe to say that an Ace or king of any suit is likely to take the trick it is played in.  As there are 8 of these a player will typically get at least 2, and bid on them.  Players receive 13 cards, so your average is 3 cards of each suit, plus one of any suit.  On a true average deal, this means that at least one opponent has 4 cards of one non-spade suit, and you will typically exhaust your supply of that suit before them, allowing you to play a spade if they lead with that suit.  Any spade suit card has a higher value than any non-spade card, so you would win that trick if you play a higher spade than the other opponent.  If you have 4 spades and 3 of the other suits, you are also likely to trump (play spade on a non-spade trick) a trick and take it.  In either of these two scenarios, a player is likely to bid on at least 1 (if not 2) low-mid value spade card.  Therefore, on average, a competent player might bid 3-4.  Now let's look at higher and lower bids from an individual, and what that means.

A low bid could be caused by a number of things:


Having multiple cards in the 2-8 range means you are unlikely to take many tricks.
Having few to no spade cards means the other 3 players have them, and you are much less likely to take tricks by your own merit in the late-game.
Having many cards of one non-spade suit is a huge detriment.  Not only do you yourself not have many spades for the late-game, but your opponents can start trumping you sooner, and if a suit is led with spades you might not be able to compete for that trick.


Now let's look at a high bid, which is basically the opposite:


Many cards in the high range increases your chances of taking a round.
Many spade cards means you can trump your opponents sooner/more regularly.  If you have all 13 spades, you take every single trick in the round.
Few to no cards of a non-spade suit is a great hand, because you can remove low value non-spade cards early in the round, and trump high value non-spade cards played by opponents.
If you play by the rule that your first card played is the lowest club in your hand, and your lowest club is 8+, it's safe to bid on that trick.


Overall, a low/high sum of bids doesn't say anything about hands, because you don't know if there are outliers.  A low/high individual bid can give you some insight as to the suit distribution and number value of cards held by that player.  If you pay attention to someone's bid, and watch the first few tricks they play, you can get a good idea of what the rest of their hand is.