SSID stands for Service Set Identifier, and is a string of up to 32 characters usually translated into a phrase or words for your convenience. Each computer in your network needs to have the same SSID in order to communicate. When connecting to your internet you may notice there are others in your range, this is where SSID comes into play, this helps to identify your network from your neighbors, if there was no SSID, then sending information over the web would be crazy. Therefore you need a SSID to identify your network from others.
There seems to be a lot of controversy about whether to broadcast your SSID or to keep it hidden. The idea behind the hidden SSID, is that the attacker won't see your network and therefore won't try and hack your system. Or it becomes much harder and unless there is a reason a hacker wants to go after you personally, they will go for an easier target. But in truth your SSID is never truly hidden from a hacker. It may be invisible to the general public, but not to your neighborhood hacker. A hacker can easily scan for networks, and a list pops up with broadcast and hidden SSIDs. So do not rely on your SSID for security. The bottom line is that your SSID should be used to differentiate between networks and not for security. To a dedicated hacker it makes little to no difference if you keep your SSID hidden or broadcast.
Most routers when first purchased have a default SSID. For instance most Linksys routers have the default SSID of "linksys". This should be changed, if not, you may be telling potential hackers that you still have all your default setting in tact. A router with all the defaults still in place, can be extremely easy to infiltrate.
Some rules of thumb when creating or changing your SSID are:
Change your default SSID as soon as possible, making yourself less of a target. Create an original, yet obscure SSID, and above all, have fun. It can be quite enjoyable creating your own unique network identity. If you need a little inspiration, you can always make a search on the web for SSIDs, often these can be quite hilarious.