There is a fine line in endurance sports between training enough and training too much. Every workout needs to count if you want to have that edge over your competition. Often though, you need to rest. You may not feel like you do today, but the quality you want out of your workout tomorrow demands it. You need to get effective rest, and active recovery will help you get there.
It is very rare that people get too much rest. This year I took a a few weeks easy, with few rides and got much stronger later in the season because of it. Taking time completely off the bike allows your muscles to rebuild themselves fully. So rest is sometimes, literally, no riding. But other times, it is active recovery.
Active recovery is getting your body moving (in the same motions that got it to this tired state to begin with) at an extremely low exertion level. If you monitor your effort with heart rate, I'm talking about Z1, and less. Your goal is to have forward motion, but you should be able to have a full conversation easily without winding yourself. There are folks who don't believe in active recovery and think that it cannot be better than sitting on your couch. I'd disagree for a few reasons.
When you push yourself and break down muscle, knots are formed in the connective tissue. These knots will remain (and continue to be sore) if they are not removed. So while you need rest to allow your muscles to be rebuilt, you also need to remove the knots from the muscles. One way of doing this is by using those muscles lightly (so as not to continue breaking them down) to get new blood in them and stretch them out. Doing so assists in speeding up your recovery. Stretching after a workout can be helpful, but as the muscles rebuild they need to be continually stretched to ensure they remain loose. One downside of static stretches are that they often don't stretch the muscles in the same fashion that our activities do, limiting their effectiveness to help rebuild the muscles that have been broken down.
Practically, if I race an endurance mountain bike race, I try very hard to get in an easy ride the next day. Moving my legs helps stretch them out exactly in the way they need to be stretched. I am
much more comfortable two days after a race doing this. If I neglect this key part of recovery, I'm sore much longer. It can also be a great opportunity to work on things like form that can be accomplished with low effort. Today I went out on the mountain bike and practiced a few things on the trails that I could still do going slow. So not only did I aid in my recovery, but I was able to use the time for other purposes as well!
When training for long races like the National Ultra Endurance Series (N.U.E.) every workout; active recovery is a vital piece to getting me back to full strength for my next tough workout.