The increase is indicated as 2sc in 1sc. This means you work the single crochets in 1 single crochet, so you get several stitches.
The decrease is indicated as sc 2 together. On the next single crochet, pull your hook through the V.L. and with the single crochet afterward by the A.L. Then pull the yarn through these 2 loops and complete the single crochet. You have then worked 2 stitches together. In the drawing, you see that you pull the loop through the entire sc; by doing it first through the front loop and the second sc through the back loop, it’s more invisible.