An anchored node is one that represents something canonically, like a place, a product, or a person. For these types of notes it makes sense to invest in ensuring that these notes only exist once and are referenced, otherwise your vault will experience fragmentation. An un-anchored note is one that just part of some unit-of-work. These notes could represent the content of a website, a log file, or a random grocery list. You might use un-anchored notes to organize or break apart content, e.g. in your Daily note.