The current reading for the sample meter here is 0369200 gallons. If the reading you had taken earlier was 0355080 you would have used 14,120 gallons.
The small red triangle shaped piece on the dial is a low flow indicator that will rotate if there is any flow through the meter. It is useful when checking for leaks. To check for leaks, shut off all the water flow in the house, such as faucets, washing machines, ice makers, etc. Watch the red triangle for at least 15 minutes. If it moves their is flow through the meter and you have water running somewhere. Another way to check for leaks is to write down the numbers and the location of the dial indicator. Do not use water for 2 hours. After the 2 hours read the meter again. If the dial has moved you have a leak and need to contact a plumber as soon as possible to avoid a high water bill.
These meters are called positive displacement meters. The water entering the meter fills a known volume of the measuring chamber on one or the other side of a movable disc that separates the chamber into two sections. As water enters it moves the disc (nutates), forcing a known volume of water out of the meter from the opposite side of the disc. The process repeats as the sections refill and empty in turn. The nutating action of the disc is coupled magnetically to the register to indicate the volume of water that passes through the meter. The meter cannot register water unless it actually goes through the meter.