Lumens are simply a measure of how bright a light is. For example, the full moon on a clear night is about 1 lumen, whereas a single car headlight is likely to be around 1200 lumens. An effective cycling light can be as little 10 lumens, however Cateye lights can go much brighter than this. The more lumens a light has the brighter it will shine.
The brightness of your lights depends a lot on the type of riding you want to do. Generally, rear lights do not need to be as bright as front lights as they don't need to illuminate the road in front of you.
For daylight riding, or riding in well-lit conditions you may not need lights brighter than 100 lumens for front or rear. It is important to remember however, that a front light of 100 lumens or fewer will not illuminate the way ahead effectively, it will simply make you more visible to other road users.
When riding in unlit areas you will need brighter lights. On well-maintained country roads, 800 lumens should be considered the minimum safe luminosity.
When riding off-road, front lights will need to be brighter still, with 1100 lumens being recommended as the minimum brightness. The uneven surface you are riding on coupled with more unpredictable surroundings makes it even more important for you to have maximum visibility. You don't want to be surprised by a low-hanging branch!
As well as the brightness of your lights, you also need to consider battery capacity and the number of lights you want to ride with.
If you are planning to ride off-road or unlit roads for several hours, not only do you need strong lights, you will also need good battery capacity. You don't want to be caught five miles from home after a two hour ride with no battery. All Cateye products have clear battery life information for all of their functions.
You may also wish to ride with multiple lights, particularly your rear lights. Many riders increase their visibility with multiple lights on different parts of their bike and person. The Wearable X Rear Light and Volt 400 are perfect lights for doing just this.