The last of the 4 principles is "Extend Ki". Understanding this principle starts with understanding what ki is, which is easier said than done. The "ki" of aikido is usually translated as "spirit" but it is also translated as "life force" and "energy". I've head ki compared to "chi" (as in tai chi), if you are familiar with that concept. In a nutshell, ki is the vital energy of life.
So what does "extend ki" mean? Well, it's basically a mental practice or a mental act that gets translated through your body. It's basically about being aware of your energy and being conscious in how you direct it. If it helps you can visualize ki as energy that starts at your center of gravity or one point and extends up and out through your body.
Another way that may be helpful to think abou it is, you've probably heard people described as having positive or negative energy, or what kind of a "vibe" they give off. Well this is gets back to how they are extending their ki, or what kind of ki they are extending.
For the sake of the principle, you want to be conscious about your energy and to be conscious in how you direct it. This can be applied to life on and off the mat, in and out of the dojo.
To your practice!