Simply Put:
Feed Rate is the speed that your bit travels in the X and Y directions while cutting and is measured in either Inches per minute or MM per minute. This will vary a great deal based on the material you are cutting / Bit size / depth of cut.
Plunge rate is simply the feed rate but applied to your Z axis. How fast your bit lowers down into your material before beginning X and Y movement to cut.
Depth Per Pass is how deep your bit is lowered into the material you are cutting for each pass. Example: You have 1/2 inch thick material and you plan to cut it all the way through. If you set your depth per pass to 1/2 inch it will take one pass to cut it out. That is not advisable for most material as it produces too much strain on the machine and can break you bit. If you set your depth per pass to 1/16 inch then it would take 8 passes to make the cut all the way through but you will get much better results.
Settings to get good results is a balancing act of getting all three set properly. There are feeds and speeds calculators available but if you use them I recommend that you be ultra conservative with your settings. The x-carve is a hobby machine not an industrial beast. That said you can achieve amazing results even when carving hard materials.
Hope this answers your question.
Charley