When you submit lessons and solvers, you guarantee that you wrote them yourself and that there is no copyright holder other than you. Otherwise there is a danger of copyright infringement.
I understand that your lessons are valuabe and that many of you have your own websites, classes etc. Therefore, you retain copyright to your work. That means that if you want to publish your own lesson elsewhere, publish it in a book, sell them, you are fully free to do so.
However, you give Algebra.Com, Igor Chudov, and Algebra Inc a full non exclusive license and rights to publish, and distribute your content. This includes your solvers, lessons, solutions and other works that you submit to algebra.com.
You understand that pieces of your solvers may be used by others. It is up to Algebra.Com to decide what exactly are rights of users with regards to copying content of the solvers. For instance, I plan on opening a foreign language sections of my site, and I will allow translating of solvers from one language to another.