When we talk about speaker measurements these terms often arise. The term free-field means that a sound source is in total open space. Nothing above, below or to the sides of it. In this total open space, the pressure level of the sound decreases by 6dB every time the distance from it doubles. That is because the area the sound energy has to fill gets bigger as we move further away from the source.
Free-field space is rarely encountered though, since most of the time we spend our lives stuck to the floor! This is known as a hemi-spherical sound field. Obviously, there is now less space for the acoustic energy to fill. Theoretically, sound level decreases by -3dB for every doubling of distance from the source. This would require the speakers to be flat mounted in the boundary, and the boundary to be 100% reflective. In reality we usually have our speakers some way off the floor, and boundaries can be absorptive covered with carpet or curtains. In an average room you will tend to see something near a -6dB decrease with a doubling of distance at high frequencies where the room furnishings will be absorbent, graduating to -3dB at lower frequencies.