Look, it's very simple.
@Manko-sensei can go over any of the below that you don't understand.
A foot is an arbitrary unit of length.
There are 12 inches in a foot. Generally inches are subdivided into powers of 2 (half-quarter-eighth-etc.), though for very small measurements mils (thousandths of an inch) may be used instead.
A yard is 3 feet.
A rod/perch is 5.5 yards.
A chain is 4 rods.
A furlong is 10 chains.
A mile is 8 furlongs, or 1760 yards, or 5280 feet.
An acre is an area equivalent to one furlong in length and one chain in width.
There are 640 acres in a square mile.
A US gallon (not to be confused with an imperial gallon) is 231 cubic inches. This can be subdivided into 4 quarts, 8 pints, 16 cups, 32 gills, and 128 fluid ounces.
A pound (avoirdupois) is an arbitrary unit of weight. It can be divided into 7000 grains.
There are 16 ounces (avoirdupois) in an pound (437.5 grains); each ounce can be subdivided into 16 drams (27.34375 grains).
14 pounds (avoirdupois) is a stone.
Two stone is a long quarter.
Four long quarters is a long hundredweight.
20 long hundredweights (or 2240 pounds) is a long ton.
25 pounds (av.) is a short quarter.
Four short quarters is a short hundredweight.
20 short hundredweights (or 2000 pounds) is a short ton.
These same grains are used as the basis for troy weights, with 24 grains in a pennyweight, 20 pennyweights in a troy ounce (480 grains), and 12 troy ounces (5760 grains) to a troy pound.
I really don't see what the issue is with wrapping one's head around this system.