The decibel (dB) is actually a ratio of two amounts power. It is commonly used to measure electrical power and the power in a sound.

If you have two power levels, say P1 and P2 then the ratio of these power levels can be expressed as:
R=10 * log10(P2 / P1).

For example, you could say that a 60W light bulb uses 10dB more power than a 6W light bulb, and a 600W light bulb uses 20dB more power than the 6W bulb but it uses 10dB more than the 60W bulb.

In the cases where a decibel is used as an absolute unit (not as a ratio), one of the power levels (P1) is an assumed reference. For example, the unit dBW means "decibels relative to one Watt." Thus you could say our 60W bulb uses 17.8dBW of power, and the 600W bulb uses 27.8dBW. dBm is another common unit which means "decibels relative to one milliWatt."

Back to decibels of sound. When people say a sound was a certain number of decibels they are implying dB SPL which means "decibels - sound pressure level." The reference for dB SPL is a sinusoid at 1000Hz which is just barely audible. Thus, 0 dB SPL is the threshold of human hearing. Therefore, saying rustling leaves correspond to 20dB SPL means that the sound of rustling leaves is 20dB, or 100 times, louder than the quietest sound a human can hear.