PLN's are personal learning networks that educators can use to develop resources and colleagues within their field to help them become better teachers and to stay up-to-date with all the current trends, both in education and technology. So teachers can become a part of a network of teachers and share their ideas, lessons, units, strategies, etc, etc.
PLN's are important because it is otherwise too easy for teachers to stick their heads in the ground and not develop any new skills or changing with the times. Which their students surely will. I was very impressed with what I saw just searching around during this presentation and I can see how this can be a very helpful resource. There are whole lesson and unit plans available and there really seems to be a sharing of teaching tools out there for us. For these reasons PLN's are important as teaching can become a solo venture in some cases. In a smaller school, you could be the only teacher of your subject. Generating your own ideas can only take you so far, but pooling your ideas with others of a similar (or different) subject area can only make you a better teacher. This falls into the theory of social constructionism, where consciousness develops in a social context. Meaning the group is not only bigger (or in this case better) than any one individual, but it is also bigger (better) than the total sum of its parts. Which is interesting to consider because that is how a classroom should ideally run.
There are numerous ways to get started on your own PLN. The first step is joining some of the online tools available, such as blogs, Ning, social bookmarking (like delicious), WIKIS, or twitter. Then start sharing your ideas, and seeking out those with ideas that interest you and adding them to your network and building on it from there.