A ETC known as an Electronic Toll Collector/Collection is used on highways/freeways to collect toll without you having to stop and pay for it. In Japan/UK and the US you pay a certain amount to the toll collection office to renew your toll collector.
For example, your on the highway and a toll tag appears in the distance you slow down before you reach the toll tag. The sensors on your windshield will be read by the toll tag's sensors which will allow for you to pass without completely stopping.
It basically saves a lot of time, does your Belta have the ETC? It's located on the outside side of the lower storage compartment on your right and the sensors are located on the windshield.
Free flow systems don't require you to stop at all. The cameras/sensors scan the sensors on your windshield while you are driving.
Here is a picture explaining the process.