I'm reading introductory material about modal logic and I'd like to be sure that I understand the basic concepts. Notably, I've read that system T for modal propositional logic has a reflexive frame. Does this mean that in T every single world is accessible from itself?
Yes. It means that given a Kripke frame $(W,R)$ with a set of worlds $W$ and a relation $R$ on $W$, the relation $R$ is reflexive, so that every world $w \in W$ has a transition to itself.