RKT is a technique for higher-dimensional Rubik's Cubes that abuses symmetry to treat a side as if it were a lower-dimensional puzzle. To learn more, see the hypercubing.xyz wiki page about RKT.
This tool specifially finds RKT cancels for 2-gen <R,U> Rubik's Cube algorithms. {n}flip means to do the rotation that looks like it swaps the R and U layers of all layers along the nth axis besides the one that you're doing RKT on. On 34, this can be notated as {1..2}OUR.