Efflux is the process of removing toxic materials from cells.
In Foldit small molecule design puzzles, efflux is a general consideration in evaluating designs. Small molecule design puzzles may include objectives related to efflux.
The number of hydrogen bond donors and the topological polar surface area (TPSA) are factors in efflux. In both cases, a smaller value is better. See Small Molecule Properties for background on these values.
Foldit Lab Report #27: Foldit gets a makeover looks at a small molecule design by a Foldit player, and discusses efflux.
See the wikipedia article Efflux (microbiology) for more on efflux, including the "efflux pumps" that cells use to remove toxins.