Definition of Waiver
"Waiver" refers to the status of a player dropped from a fantasy football team in your league. During the fantasy football season, it will be necessary to make changes to your roster. When you drop a player (usually by clicking on a "Drop Player" link on your team page) that player is immediately placed "on waiver".
This initiates a waiver period (between zero and seven days depending on your league) during which anyone in your fantasy league can claim the player.
Here's an example. Say you drop Joe Flacco on Monday afternoon (very likely this year). All the fantasy managers in your league can make a waiver claim on him until the waiver period passes -- in my leagues that would mean the waiver period would end two days later, at 11:59 pm. That player is then assigned to a new fantasy squad based on that team's waiver priority or if he is unclaimed by everyone, he becomes a free agent. Free agents are available instantly without a waiting period of any kind.
Here's the trick part -- if a manager adds and drops a specific player in the span of 24 hours, that player doesn't move to the waiver list, but instead is instantly made a free agent. This is important to remember when cutting players from your squad. Are you accidentally handing a free agent to your competitors? But this 24 hour rule exists for a good reason -- it prevents managers from "locking" up players that would otherwise be free agents off waivers so that the other managers in the league don't have waiver access to them and cannot add them to their squad.
Check the waiver period in your league. Do it right now. Understanding the waiver process for your league is the first step to getting ahead of your (hopefully lazier) competitors.