Answer: Up1 is a way to increase the number of stitches. When increasing in this way, you don’t make new stitches by working into the stitches already on the needle.
Instead, with the right-hand needle, pick up the loop lying between the stitch you’ve just worked and the next stitch and place it on the left-hand needle.
Knit or purl (as required) through the back of the picked-up loop. As you work into the back of the loop, the loop is then twisted, which will prevent holes and gaps forming.