That is what is called the Depth of the Order Book, or the bid - ask spread in a user-friendly presentation (much like the Order Book itself).

Any order, whether a buy or sell order, can be canceled or placed at anytime, further adding to the volume at a particular price point on the depth chart. Some traders and/or investors will use Stop Limit Orders or Trailing Stop Loss, in which their sell orders could exist over a ‘range of price points’. Keep in mind that not all orders are visible on the depth chart.

One side being bigger than the other, at just a glance, means nothing, as orders can be placed and canceled at anytime. BUT....the depth provides you with a visual aid and stats that reveal the ‘volume’ at particular price points. This can be extremely useful when intraday trading.