Hi Henni,
Your interpretation is correct. This prefix before the colon ensures that the IDs are unique if more than one feed defines the same IDs. It is very common for IDs to clash since many feeds assign their entities sequential numeric IDs like 1, 2, 3 ...
This disambiguating string is referred to as the “feed ID”. You may also see people refer to it as the “scope” of the ID. This is always a number when it is automatically generated by OTP, but you can also control the feed ID by adding a feed_id column to the feed_info.txt file within your GTFS feed. In that case, the feed identifier can be a sequence of any characters except a colon or an underscore, typically only alphanumeric characters.
It wouldn’t be necessary for OTP to add this prefix when there’s only one feed loaded, but there’s some value to having consistent behavior in every case (one feed or many feeds).
Hope this helps,
Andrew