A steek is a column of extra stitches added to a piece of stranded colorwork knitting specifically so you can cut it open later. You knit the whole thing in the round, beautiful and intact, and then you take scissors to it. On purpose.
The steek exists because some structures need to be built closed before they can be opened.
Knitters who steek for the first time describe it in one of two ways: terrifying, or the most liberating thing they have ever done with a pair of scissors.