Post stitches are stitches worked around the post of a stitch, usually a double or treble crochet. It's sometimes difficult to get a picture of the different steps since I have only two hands and need them to hold the camera. LOL I suggest doing a search at YouTube or other online resource for videos. Nexstitch has a good one you can look at.
Front Post Double Crochet (FPDC)
Instructions: yo, insert hook behind post of stitch (from front to back), yo, pull through, bring up even with last stitch completed, yo, pull through 2 loops, (2 loops on hook), yo, pull through rem 2 lps to complete stitch
Here is the video demonstration
Back Post Double Crochet (BPDC)
Instructions: yo, insert hook into the back of your stitch (in other words from back to front), go in front of the post, pull through (3 loops on hook) bringing stitch up to height of previous stitch, yo, pull through 2 loops (2 loops remaining), yo, pull through remaining 2 loops to complete stitch