It's where the participants each make an entire row of blocks, then all the rows submitted by all the participants will be sewn together to make the top. There is a time limit to complete each row before it is passed onto the next participant until all have made the specified number of rows.
Whoever organizes it determines the size of the blocks that make up the rows, which of course sets the row size, as well as stating the colors, theme-if so desired, and any other requirements for the project.