Recently someone mentioned a piece of equipment to measure distances. They stated that it was used in military. I think it was called "pace counter". What exactly is it and how does it work?
A pace counter is a low-tech, manually operated, very light-weight device that you can make yourself from heavy cord and beads, or buy for $5 to $10. In contrast to a pedometer, which counts for you, you have to mentally keep count of the paces you take; there is nothing automatic about it.
This article tells you how to make your own. The article calls the device Ranger Beads and says they were originally developed by the military.
Wikipedia tells you how to use the device.
The tool is usually constructed using a set of 13 beads on a length of cord. The beads are divided into two sections, separated by a knot. 9 beads are used in the lower section, and 4 or more beads are used in the upper section. There is often a loop in the upper end, making it possible to attach the tool to the user's gear with a simple Larks head hitch.
How to use
There are two ways to use the beads. One is to represent the paces the user has walked, while the other is to represent the distance walked.
Both methods requires the user to know the relationship between the paces walked and the distance travelled.
As users walk, they typically slide one bead on the cord for every ten paces taken. On the tenth pace, the user slides a bead in the lower section towards the knot. After the 90th pace, all 9 beads are against the knot. On the 100th pace, all 9 beads in the lower section are returned away from the knot, and a bead from the upper section is slid upwards, away from the knot.
In this manner, the user calculates distance travelled by keeping track of paces taken. To use this method, the user must know the length of his pace to accurately calculate distance travelled. Also, the number of paces to be walked must be precalculated, or the distance travelled has to be calculated from the walked paces.
For every 100 metres the user walks, one of the lower beads is pulled down. When the ninth of the lower beads is pulled, the user has walked 900 metres. When the user has walked 1000 metres, one of the upper beads is pulled down, and all the lower beads are pulled back up.
Using this method the user must know the number of paces walked in 100 metres. An experienced user can also adapt the pace count for each hundred metres depending on the terrain. When using this method the user does not have to calculate or look up how long a distance to walk or the distance travelled.
It sounds like you are referring to a pedometer. These things basically count footsteps or strides. The purpose is usually to infer distance walked or hiked, although there is some error in going from number of strides to distance.
Old style all-mechanical pedometers used a small weight to sense vertical acceleration. The weight going up and down advanced a counter, with the result usually displayed on a dial. Modern ones use electronic accelerometers, but still basically do the same thing.
If you just want to know distance covered, a GPS with tracking capability will usually yield a better answer. Note that this capability is available in tablets and smartphones, with the right software app. Such tablet and smartphones also have accelerometers (this is how they switch between landscape and portrait depending on how you hold the device), so if you really want a pedometer, it can be a simple as running the right app on such a device.
You could even run a GPS tracker and pedometer together, then compare results on some known path to see what to believe from either in the future.