More or less, ratio deco is identifying trends in decompression and utilizing those trends to calculate your decompression. Often, it is combined with other things (such as a reshaping of the ascent curve) and utilization of the oxygen window.
You'll often hear of things like "1:1" and "setpoint". These terms refer to the baseline of the ratio. For instance, at 150ft on 21/35 (oxygen/helium percentage), there is a 1:1 ratio between bottom time and decompression time when using 50% oxygen as a deco gas. This ratio breaks down at a certain point, and is not the end all be all of deco. If the dive is deeper or shallower than the setpoint (1:1 at 150), the deco time is modified by simple rules of thumb that mirror what a decompression algorithm would put out. Different depths get different gases and different setpoints.
Oh course, there are assumptions that go along with this, such as standard gases, proper familiarization with the decompression theory, ability to do mathematics underwater, and most importantly, knowing WHY you're doing what you're doing.
Personally, I use ratio deco as a backup to some modified tables I made. Others use it as a primary source of deco information with great success, and others use it without being very successful.
