In the AD&D 1st edition monster Manual, every monster's speed is represented in inches. Obviously, a big monster cannot just move 6 inches per turn, so what does the " mean?
In AD&D 1st edition inches of movement represents three things.
6" = 60 feet per turn exploring a dungeon. This allows for the normal checks for surprise, mapping, detection of secret door, etc.
6" = 60 yards per round moving through passageways. Basically if the person or party is in a interior location that they know they move at this rate.
6" = 60 yards per round moving outdoors like in a city.
6" = 6 miles per half-day treking.
This is all found on page 102 of the AD&D players handbook.
And to be complete for range (both spells and missile weapons) there is the following.
1" of range = 10 feet indoors 1" of range = 10 yards outdoors
This represents the ability to lob missiles in an arc outside as opposed to a flat trajectory indoors.