Audio crossover is an electric circuit that separates low and high audio frequencies in a signal. It is needed because very small loudspeakers (tweeters) would break if fed by low frequencies. Likewise, bigger speakers (subwoofers) are not able to play higher frequencies purely.
Audio crossovers are divided to passive or active crossovers. Typical components used with passive crossovers are capacitors inductors and resistors. If passive crossover uses resistors to adjust loudness, those generate heat. Instead, active devices typically use resistors and capacitors with other audioelectronic components and those typically don't consume so much electric power. Also, active crossovers are easily built to be adjustable. Still, that doesn't mean that using passive crossover means low quality.