"BSF Lessons" are Bible Study Fellowship classes, which are interdenominational in-depth Bible studies taking place in organized small group settings around the world. The BSFInternational.org website has a search feature which allows people to locate classes nearest to them and sign up to join.
The Bible Study Fellowship lessons are courses that are 30 weeks long. Study questions for the lessons are available only to class participants, but the website does offer "home discussion pages" for two of its study series free to anyone. There are also "home training lessons," which are articles to help parents apply biblical truths to various parenting situations. The home training lessons are free and available to anyone.