What does M1 means in shell pattern

I’m a very green beginner and I thought this pattern would work for me, but I’m confused with the pattern directions. The pattern came from Free-Knitpattern.com and is called Easy Shells Baby Blanket. The pattern stitch goes as follows:
Cast on 105 sts
Row 1: Knit
Row 2: K3, p across to last 3 sts, K3.
Row 3: K3, [p2tog] twice, [M1, k1] 3 times, M1, *[p2tog] 4 times, [M1, k1] 3 times, M1, rep from * across to last 7 sts, [p2tog] twice, k3.
Row 4: Rep Row 2.
Rep Rows 1 - 4 for pattern

My confusion is the M1.

M1 means Make 1, it’s an increase.
I guess you can use your favorite increase method.

M1 is an increase done between 2 sts, not a kfb. Look at the increases page for a couple ways to do it. You can pick up the yarn between the 2 sts and knit it through the back loop, or use KLL which picks up the back loop of the st below the one on the left needle and is knit into.

A simple backward loop increase will often work well as a M1. This increase is also called an e-loop increase sometimes, maybe has other names too. Try it in your particular instance and see if it works (I suggest a swatch so you can try it over your whole pattern repeat) well. It is easier than most M1s.