Purl front and back (pfb, pf&b, or p1f&b) is a one-stitch purlwise increase usually done on the wrong side of the fabric, but sometimes done on the right side. The purl front, back & front (pfbf or pf&b&f) is a 2-stitch purlwise increase also usually done on the wrong side.
“Knit in the front and back” or “knit front back” (kfb) is a common increase in knitting. This increase leaves a small purl bump to the left of the stitch. If you don’t want that purl bump in your work, An alternative to the kfb is the “knit front back slip” (kfsb). Learn how to knit both of these increases in this tutorial.
You don’t need to unknit or rip out your work to fix a forgotten make one. Learn how to fix M1L and M1R in this tutorial.