It refers to a more advanced concept/theory of tissue nitrogen loading and off-gassing. It basically theorizes that the body's tissues can be categorized into tissues that quickly absorb and off-gas nitrogen, and tissues that more slowly absorb/off-gas nitrogen. Each type is a factor and consideration for gas and decompression planning for advanced dives.
You are not the only one out there that would be clueless coming across a discussion referencing this. It is not really covered, or covered well in OW and AOW courses, as the dives one is trained to do with these levels of certification are non-stop/no-deco diving. Theoretically, if diving within recreational limits, table limits, etc...then one could surface without much, if any, consideration to actual type of tissue loading.
If you read/follow discussion about deeper or lengthier dives that take the diver beyond recreational limits, particularly with advanced gas mixes such as trimix, heliox, etc., then tissue loading as part of decompression theory becomes particularly important.
Other than that, it is good info to know, as it makes one a more competent diver, regardless of certification or experience level. But for most of the mass diving public, it is not really covered until they begin more advance training such technical diving, advanced nitrox and decompression procedures (ANDP), and penetration diving (cave/wreck) to name a few.
-Z
