passband efficiency and stopband rejection

[karambos]

I'm the proud owner of a pair of Genelec 1029A's and I'm looking into the possiblities of buying a subwoofer in the same range.

Everywhere I look I find the following sentence and I've no idea what it means:

"The 8" driver is front-loaded to improve both the passband efficiency and stopband rejection"

Can anyone enlighten me as to what "passband efficiency" and "stopband rejection" are?

thanks