Passive pickups don't drive a signal too well down long cables, resulting in a loss of the high frequencies. Once the signal passes through a transistor gain stage, it is 'buffered' and can be driven down a long cable without much degradation. This is the basic premise. So, active basses have a built in buffer. Some effect pedals are true bypass, meaning they are effectively out of the chain when bypassed, but some pedals (most older pedals, plus all current Boss and Tech21 pedals for example) are buffered bypass. So if you have an active bass, or a pedal with buffered bypass early on in the chain (e.g. a Boss tuner) then you can add loads more pedals without hearing much of an effect. However, not all buffers are well designed, and some can noticeably colour the signal. So a good rule of thumb is to have one good quality buffer as close to the pickups as possible, then use true bypass pedals for the rest of the signal chain!