Have you ever taken a look at all the personal wireless networks around your home? If you live in a house, you might notice a couple of your neighbors' networks. But, if you live in an apartment or condo, you could easily be within range of ten or more wireless networks.
Each wireless network can be given a specific name to identify it. For security reasons, wireless networks can also be configured to not broadcast their name, but people rarely do this on home networks.
Since your network is broadcasting its name, people can see it. If your network requires a password, then they can attempt to join it by repeatedly trying different passwords.
To avoid this, I've given my home wireless networks names to dissuade people from joining them by naming them with financial rates to look as if they're metered: $4.95/minute is the name of my Airport Express network, and my two home base station networks are named $9.95/minute and $12.95/minute.
Over the years I've had a number of guests over to my house who would try to join my neighbor's wireless networks before asking me the name of my network. They've always had a good laugh when I told them the name. They usually comment that they were trying to access all of the other networks and stayed away from the $12.95/minute network just in case it could some how run up a bill on their device.