Little: In the case of me (and some friends, and this story thing) a little is someone who goes into littlespace. For some people, it's a kink, but for me, and the characters in this book or whatever, it's completely sfw and mostly a coping mechanism.
Littlespace: A headspace that involves mentally regressing, or feeling younger. Some go to nearly newborn, while some go to more of a teenager age.
Age Dreaming: Like littlespace, but not regressing completely. For example, doing homework or cooking while listening to something that makes you feel small and sucking a paci. You're not regressed but you're not 100% big.
Go ahead and ask whichever questions you would like!