Hi, welcome to KH. Assuming you marker is on the needle between two stitches, you just move it to the other needle. If you look at the top of the page here you'll find a Glossary button. There are lots of knitting abbreviations there, some have videos attached.
What are you making? If you have any problems, this is the place to come for help. We have some of the best knitters you'll find here. They're great.
I should never overestimate my basic arithmetic skills.