Boost is a dual-band, parallel compression effect that enhances your audio by individually boosting the lows and highs whilst still letting some of the original signal through to get a nice blend.
It uses the same guts as bathtub, configured as a dual band compressor (high and low band), with dry audio also mixed in parallel. This gives you a pleasing mix, keeping the transients and mid range of the dry audio, mixed with the compressed highs and lows to boost that kick and make the high hats shimmer.