You are supposed to have a little round marker on the needle at this point (a bit of waste yarn tied into a circle works well- put the needle through the center of the circle so that it slides along the needle with the stitches), probably before this somewhere you had the instruction to PM (place marker). When it says slip marker you just move the marker from the holding needle to the working needle, again having the needle go through the center of the hole. This will mark where the increase is.
I have a pattern that says "Rnd 3: K to marker, slip marker, m1, k to next marker, m1, slip marker, knit across."
I don't understand how to do this!
You place a marker which can be a contrasting color by tying a short length of yarn around the needle and then slip that little bit of yarn to the needle you are transferring stitches to, then increase one stitch by lifting the bar between the stitches, and knit into the back of this new stitch , that is the increase of make 1, knit to the next marker do the increase again, slip the marker and knit across the rest of your stitches. It sounds like you are getting the stitches set up to knit a thumb gusset in a pair of mittens.. Youtube has a lot of knitting videos you can watch on knitting thumbs in mittens.
Daylilies are the Lord's smiles, a new one every day.