This is your standard-issue picture of the acupuncture meridians and acupuncture points. Many of us have seen this picture, or something like it, hanging around. The lines are the meridians and the Qi (or that stuff that means the difference between being alive and being not-alive) travels through them. The dots on the lines are the acupuncture points and we can access the Qi flowing through the meridians at these points. But still, how does it WORK?
I like to explain to people that meridians are like a freeway system and our Qi is like the cars on the freeway. When we're healthy and pain-free the cars are driving around fine, there's no traffic, no accidents and no road-rage. It's when accidents (a broken bone, a bruised elbow, an arthritic knee), the evening commute (an imbalance of Qi with too much in one meridian, and not enough in another), or road-rage (stress) occur that create symptoms in us.
For an accident example, you sprain your ankle playing basketball and it's doing all of the classic things that ankle sprains do: the area starts swelling (the Qi cars are getting backed up!), it is painful to the touch, let alone to walk on (the people stuck in the traffic jam are starting to get cranky!), and it's turning all KINDS of fun colors (the Qi cars aren't allowed past the accident). For treatment, an acupuncturist is going to play the role of the Department of Transportation. They are going to open the meridians and move blood (get the Qi cars involved moved to the side of the freeway to allow the cars to pass) and remove stagnation (get some of the Qi cars to exit the free way and take another route). All of these actions will allow for the swelling to come down and for the Qi to get to and from the area in a proper, organized fashion to promote healing.
For an evening commute example, you're exhausted constantly and really have a hard time getting yourself to do much of anything. You don't have quite enough Qi to spread around (there are too many cars on the road, instead of working or running the house - doing the things that help our society function), so you see a noticeable decrease in your ability to multi-task. Multi-tasking here includes the action of being alive, not just all of the things we say we're doing when we're multi-tasking such as exercising, painting the house, making dinner, keeping the kids entertained, etc. An imbalance is created because we need to use our Qi to stay alive and keep our bodies functioning, but there's not quite enough left over to do the fun stuff (or the not-so-fun-but-necessary stuff!). Treatment will be to increase your amount of Qi, while making sure that it is distributed properly. If our Qi levels aren't kept high and the distribution is uneven for very long, our bodies will start cutting back on its functions - this is when unhealth happens.
For a road-rage example, you're stressed out. Work has been too demanding; NO human can do what they're asking! You think and worry about it incessantly and it's even disturbing your sleep! Stress really messes with us, yeah? So what's going on energetically is that when we get stressed out, our energy flow becomes ragged instead of smooth (erratic driving), which then leads to getting irritated easily (that driver sure seems angry!) and our Qi going against its normal flow (the one committing the road-rage may even go so far as to follow someone off an exit that wasn't in their pre-planned route!) often leading to sleep disturbances, heart burn, appetite changes and such. Acupuncture will help calm us and promote the proper movement of Qi, allowing for health and happiness to return.
So in short, Qi is what makes us alive, the meridians are the pathways that Qi travels through to get to the different areas of our bodies to do it's work and acupuncture points are the access points to influence our Qi and its proper amounts, distribution and flow.