Blocks are a navigational tool in your topic and can provide quick links/access points for students. (However, be aware that if a student is using a mobile device to view the topic, the blocks will drop to the bottom of the screen.) Students can also interact with blocks in your topic by docking them.
Blocks appear in the left or right of the topic screen in FLO. "Topic links" is a standard block and a WebPET feature, with links to topic information, library resources, grades, SETs, etc. Different kinds of blocks can be added (e.g. Activity, Topic welcome, Remote RSS feeds), or you can add an HTML block and use it to feature the textbook/s or for some other topic-related purpose.