Actually ... it means that your carrier has no tower wherr you are, but they have a contract with a carrier to allow their customers to use the tower of that carrier. The only reason it shows (unless that's still the practice in other countries - it's no longer the practice in the US) is that you used to have to pay for roaming, usually by the day (in other words, if you used X carrier's tower for one call, one text or downloading one email, you'd pay for a day's usage [as well as for texts, which were charged separately, and it counted against your monthly limit of calls - no unlimited voice back then].)
These days, in the US, it's about as important to you as the temperature inside Vesuvius at the moment..