Volume refers to the total number of shares/contracts that have traded hands over the course of the day. 

Volume values update LIVE intraday.

The volume value DOES NOT differentiate between bought or sold options; ANY traded contract will count as 1 volume. 

Volume Resources:

The Hottest Contracts feed can be configured to display the contracts with the highest volume

The Contract Look Up page displays historical volume and open interest data

An analogy for Volume and Open Interest on Twitter

Tracking whales using Volume and Open Interest on Youtube