Upon importing a sheet, you will be asked how manny columns and how many rows.
These simply relate to the images from left to right, and how many of these rows below each other.
thus, you will have rows with the same amounts of images.
The value you use for the columns is the number of images used to split the total width of the sheet in. (images per row)
The value for rows is the amount used to split the total height of the sheet, automatically setting the height for all the images per row.
You can also simply do 1 row, with x amount of images, all up to you.
Simple ? no ... took me some too xD
best example showed me:
Make a square image, and place an image in each corner.
import sheet: 2 columns, 2 rows
each frame will be one of the corner images.