I need the definition of "Head Feet".

I think the circulator on my system may be too small. Looking at the TACO 0xx-F5 data sheet, the graph show GPM vs. Head Feet. I think I understand the graph. I'm just not sure what Head Feet refers to.

Also, I need to know the recommended flow thru the following Buderus boilers:

G234x-45
G124x-25