Nicht Shabbos Geret is what you would say of something that is not halachically suitable to be spoken about on Shabbat. Of course, it should be said gently, if at all. For example, someone remarks, I heard youre selling your car. You answer (optionally tapping the side of your nose), Nicht Shabbos geret. (Theres a famous joke along those lines, the punchline of which is I sold it already, nicht shabbos geret.)
The phrase is Yiddish/German. Geret is the past participle of reden, meaning to speak. The idea is based on a verse from Isaiah 58:13, from which the rabbis learn that just as ones actions on Shabbat are different, so ones way of speaking on Shabbat should be different from weekday speech. This is a helpful way of relaxing more on Shabbat by neutralising weekday thoughts and worries.
The general rule is that anything which cannot be done should not be spoken about. Saying that you plan to drive to Brent Cross tomorrow is nicht Shabbos geret. (However, saying that you will go to Brent Cross is not, if its conceivable that you could walk there.) There are some who say that mundane and idle talk should be minimised on Shabbat. That is a high level to achieve, but the aim is to make Shabbat a time for more meaningful conversation.