I try to purchase a set of speaker and found the description of active subwoofer and passive satellites. What do the active and passive mean? Thanks.

An 'active' speaker has an amplifier built in, so all it needs to make a sound is a 'line level' (low power) signal from a surround receiver. 'Passive' speakers have no in-built amplifier, so they require a power amplifier to drive them (with a higher voltage and current than a line output).

If you look on the back of a surround receiver, you will see that (usually) there are 'heavy-duty' terminals for the forward and surround speakers, since these are driven with considerable current from the amplifier, but the sub-woofer output is usually an RCA ('phono') connector, because it provides only a small voltage, with negligible current to a powered (active) sub-woofer.

Daniel