Lets start at the beginning.
You cast on 15 stitches and knit 2 rows. When you knit every row you are doing garter stitch. They want you to keep an edge stitch on each side in garter stitch so on every row you knit the stitch at each edge. You follow the M.1 chart for the center 13 stitches. 1 edge stitch on each side + the 13 for the chart=15. Starting here they want you to increase 1 at each edge every other row by doing a yarn over. So row 3 will be: K1, yo, chart M.1., yo, k1.
it said "Inc inside 1 edge st by making a yo. On the return row knit yo into back of st in order to avoid a hole" what do i need to do, how to yo into back of st , i know how to yo by watching the video but just not sure yo into back st, is it the same thing?
When you do row 4 you will be knitting the yarn overs you made on row 3 in the back to twist them and close the hole a yarn over usually makes. You are not doing a "yo into the back of the st", you are just knitting the yarn over from row 3 in the back. Here is how you do row 4: K1 (the edge st), and then knit the M1 chart starting at the left edge and working to the right. Evidently the first stitch is a knit. To knit it, instead of putting the right needle where you usually do to knit, put it behind the left needle, straight on pointing right to left into the strand of the yo, wrap your yarn like normal and knit the stitch there. Continue across the row and when you get to the yarn over on the other side work it as I described at the beginning of the row, knit the last stitch.
You keep increasing every other row like that until you have 27 stitches. The stitches that you increase on each side of the 13 chart stitches they want you to work in Reverse Stockinette. So just like regular Stockinette, but with the back side as the right side in this case, but keep the 1 stitch right at each edge in garter by always knitting it.
Knit to 65 cm. Now begin the decreases.
And then : "Dec inside 1 edge st" , what is it mean?
That just means to work the edge stitch on each side and work the decrease as it says:
Dec as follows after 1 edge st: P2 tog.
Dec as follows before 1 edge st: P2 tog into back of st.
in one stitch from the edge.
So at the beginning of the first decrease row: K1 (the edge stitch, P2tog, work across to the last 3 stitches P2tog into the back (also called p2tog tbl--through back loop), k1. If you don't know how to p2tog tbl look that up in the free videos. (Here is my description of how I do it: when you have 3 stitches left on the left needle, keep the yarn in front because you are purling, twist the left needle tip in toward your body until the back of the work is showing, insert the right hand needle tip right to left through the second stitch from the tip and on through the first stitch, let the left needle go back to the normal position, yarn around and purl those 2 stitches together from that weird position.) Be sure you have the yarn in the back for all knit stitches and bring it to the front between the needle tips for all purl stitches. To change from a purl to a knit again, take the yarn back to the back between needle tips.
There is no information about where this pattern is on your post. I am quite certain it is a Drops pattern from GarnStudio. I will be able to find it if you give the pattern numbers. It should be some number with a hyphen in the middle. I will be happy to look at the chart with that information.