Tsit5 - What is it?

I did some poking around – I think this is just a particular implementation of runga-kutta 4(5). Is this correct?

Yes, see the (brief) docs. According to the underlying paper and Chris’s benchmarks, it hits a sweet spot in precision and speed for many non-stiff problems using RK4(5).

It’s a newer tableau of coefficients for Runge–Kutta by Tsitouras (2011) that is more efficient than older versions like Dormand–Prince, as reviewed here by @ChrisRackauckas.