Cross training refers to the action of training in a sport that is not the one that the athlete competes in. If you have made your way here, you will consider yourself or at least be thinking of calling yourself a runner! For any athlete that is a runner, cross training refers to training deosn't involve a weight bearing running action. The weight bearing part of that sentence is important!
The goal of cross training is to improve your overall performance.
Fortunately, or unfortunately (depending on how you want to look at it), it is not possible to run everyday. That isn't strictly true but you increase your chance of injury by just performing one action i.e. running. If you are able to spread the cumulative level of orthopaedic stress over a range of difference muscles and joints, you will be able to exercise more frequently and for longer durations cumulatively during the week.
When you train, you heart doesn't know what you are doing. Whether you are cycling, lifting weights or jumping rope, the heart muscle reacts the same way. In that respect, cross training is great and it will keep you fit.
As a runner, there are forms of cross training that are more applicable to running than others. In list of decreasing importance you have
The closer we are able to mimic the running action, the more useful the session will be in improving your running. However, you can see that for these activities, your body weight is supported and you don't get the impact forces that are associated with running.
What we are doing when we are cross training is reduce the impact that your joints receive when you are running but we are still training to maintain or improve fitness.