How do I "inc 1 st in next st"

I am making my first pair of mittens and starting the thumb gusset it says "“inc 1 st in next st”. I am not sure what that means…do I do a YO or a M1 or kfb or is this something else? If anyone knows, please help.

With many thanks!
Kim

aliekitn99 on ravelry

Use kfb when a pattern says to inc IN a stitch. A yo or m1 put the inc between sts.