It means that, in some unspecified way, you are like a potato in a sack. You should probably ask the person who told you this in what particular way they feel that you resemble a starchy root...
A potato is a very smart vegetable, it can hear you when you talk to it
Its when Chuck Norris "T-bags"
It means your gonna get layed
Usually means "get out of bed", particularly after intercourse.