There are two main reasons to use the word "foot."
First, it’s a part of the body.
This is a foot.
These are feet.
People usually wear shoes on their feet.
If they don’t wear shoes, they go barefoot.
I like to walk around the house in my bare feet. (no shoes.)
The second reason for using the word "foot" is as a unit of measurement. Most people in the United States aren’t very good at measuring things with the metric system. Instead, we use inches, feet, and yards.
An important thing to remember when measuring things is that units of measurement are often combined with numbers to describe length, height, and distance. When this happens before a noun, don’t use the plural form.
The table is eight feet long.
It’s an eight-foot table. (Notice the use of the hyphen between "eight" and "foot.")
The ceiling is eight feet high.
It’s an eight-foot ceiling.
The water in the lake quickly drops down to 20 feet. (drop = go down)
There’s a 20-foot drop in this lake.
Do you understand the differences in the sentences above?