It's simple really. When sound hits a wall, some bounces back, some doesn't. The unbounced sound either gets lost in the wall or passes through to annoy the next apartment, but from the point of view of the source, it's absorbed. The fraction of the sound that is absorbed (in decimal form) is called the coefficient of absorption. An open window has a coefficient of absorption of 1 at all frequencies. Anything else has a value less than 1 that varies with frequency.

Some Examples:

Total absorption is measured in Sabines, and is calculated by multiplying the coefficient of absorption by the area of material in square feet.