It means “Work the stockingette stitch for five inches.”
Stockingette stitch is knit 1 row then purl 1 row. Repeat. The right side will be smooth and the wrong side will be lumpy; your work will tend to curl with the purl/lumpy side outside - this is normal.
These instructions mean just do your knit 1 row, purl 1 row for five inches without increasing or decreasing - in otherwords, the side edges are “even”.
In fact, that’s another way to say this: work st. st. even for 5" [work the stockingette stitch even (not increasing or decreasing) for five inches.]
Hope this helps,
PS - Welcome to the Wonderful World of KNITTING! :woot: