What does playability of a Scrabble word mean?
"Playability" refers to how often these words crop up in real games. Someone will have made a database of all the words played across a large number of "high-standard" games (e.g. club level and above) and ranked how often certain words crop up. 

QIN ranks highly because it is a very common way of getting rid of a Q (since there are relatively few Q words to choose from). In addition, it can be formed by hooking -N onto QI, which itself is a very commonly played word. Thus QIN can be considered a useful word to know.

In your example, it might be quite common to have the option of playing ERE, but in most situations that won't score you many points, and there will be better combinations present on your rack.

Note that "playability" is a slightly more subjective concept than "probability". The latter can be calculated precisely, whereas playability will depend on the set of games chosen to average across, and ultimately the decisions of the players taking part in those games.