Sometimes it's about roleplay. Sometimes it's more how you feel or how you identify...Basically, though, Kevin has it right, and it usually shows up in a D/s context in which the Dominant partner acts as a parent, in a way, while the submissive is the child. The "little" wants/needs to be cared for the way a child would be. It is emphatically NOT a sexual kink; when in "little" mode, the person is treated exactly as one would treat a real child the age the "little" identifies as. For example, the Dominant might give the "little" coloring books and crayons, or read them a bedtime story, or brush and braid their hair, or that kind of thing.
It's hard to explain thoroughly, and attempts to google the definition I'm trying to give were unsuccessful. LittleStuffies, one of our newer members, has a blog thread in which she discusses being a "little", so that's a resource that might explain it. LovingRadiance also references the concept in her blog.
In my case, *I* am not a little, but because of the traumas in my past, fragments of my personality essentially broke off and froze at the ages I was when the most severe traumas occurred. As part of my therapy early on, I was told to address those aspects as if I were speaking to *myself* at those ages and tell "them" the abuse wasn't their fault...and I couldn't do it. I was too angry and too hurt, because I was only just realizing that all those things were WRONG, and only just accepting the impact those things had had on my life to that point (I was 36; it was after I left my kids' father), and even though I knew the abuse wasn't my fault, I was furious with the "children" pieces of me.
The therapist asked if I would be angry with a real child who had experienced trauma, and I said of course not. The problem was that I was angry with myself for being "fucked up", and I focused the anger on the fragments of myself left from those times. So the therapist suggested addressing the fragments as if they were separate children, and that worked. I still do it sometimes, though of course they're not separate, because sometimes it's still a therapeutic thing.
Because of all that, sometimes instead of feeling like the 45-year-old me, I feel like the 4-year-old me. Or the 6-year-old. Or the 8-year-old, or the 11-year-old. I don't *act* like it such that anyone would realize that's what's going on, but some of the people I've known, like Woody and S2, recognize it anyway. Woody can tell because I told him about "them" the day we met; I never figured out how S2 was consistently able to engage with "them" even though he didn't know "they" were there.