Do you mean 1T/2T for memory timings? It's the command rate. 1T is faster by approx 5%, and by faster I mean mostly in benchmarks. It has very little effect on gaming.
Sometimes you have to loosen the other timings to achieve 1T because it can cause crashes if it's not stable. Try it, if it works great, if it crashes revert! Won't hurt.
for the older systems like the LGA775 (I used to have a Q6600) switching the command rate should be just fine. With new Ryzen systems, not so straight forward.