Also known as "stockinette".
Pretty simple. When knitting flat: knit one row, purl the next. When knitting in the round: knit every row.
This makes a fabric which is flat on the "right" side and bumpy on the other (if the "other" side is deemed to be the "right" side, then this is called reverse stocking stitch).
Stocking stitch tends to curl at the edges; the cast-on and cast-off edges curl upwards while the edges formed by the starts and ends of rows curl backwards (see photo). You can overcome this by adding a border of something like [moss stitch]?, rib, or garter stitch.