Wireless Internet on HP laptop

So, my brother recently bought a HP laptop that has build-in wireless connection but the thing is that we don’t have an internet connection yet. However he tried connecting to a network and it showed a list which had a few secured networks and 1 unsecured network. We connected to the unsecured network and we could connect to the internet. Now the question is, are we using someone’s elses internet in the neighborhood? And is it safe for us to use that, because I don’t want someone to be sending us any viruses and stuff.