What is R1?
The first resonance frequency of the vocal tract. Primarily corresponds to the volume of the pharynx. — Scinguistics' Shared Glossary
R1 is a filter on your voice that is the most important factor of "vocal sex". It's frequency can be heard by flicking the throat, but this sound disappears during speech, but produces a "loud point" where frequencies closer to your R1 are amplified, producing a more feminine or masculine sound (depending on how it is modified).
A factor of R1 can be visually seen too (or at least felt with touch), as the larynx should be able to be seen rising and falling during speech and R1 manipulation.
More info & Exercises
Z released a great video on R1, including exercises + tons of info!!