A row is a horizontal line of positions across a slot's reels. Grid sizes are written reels × rows, so the common 5x3 layout has five reels and three rows, showing 15 symbols at a time.
Rows matter for how wins are counted. On payline games, lines run through set positions across the rows. On ways-to-win games, the number of rows on each reel sets the number of ways: a 5x3 game has 3^5 = 243 ways and a 5x4 game has 4^5 = 1,024.
Some games change the number of rows during play. Megaways titles vary the symbols per reel on every spin, and games with grid expansion unlock extra rows during features, which increases the ways or lines available.
Row count says nothing on its own about how generous a game is; the paytable and maths model decide that.