You need a license in order to handle steam, obviously. I can tell you're new, but that's okay. First, you must get a state registration from your nearest steam handler provider and apply for an application, and in 10-30 business days they'll tell you whether they accept you or not. After that, you just have to go at an appointed time for the steam test, and if you pass, you get an honorary ID that allows you to handle steam in a safe and professional manner.
It's a license to work with steam or steam operated machinery, such as trains. It's used to help further identify you. So, for example, when you work at a train station and you have your SteamID you can use it to find coworkers at other stations. Or, you can use it for ID purposes like - same as a passport or drivers license.
I knew a guy named Jimmy Joe Bobby Smith Helms VI and he got his SteamID from the SteaMV (the DMV for steam) and he was able pursue his dream in life to handle steam and use the energy force to perpetuate the American economy to a more efficient future.
lol no seriously. Steam is a game software that allows you to buy and play games. The ID bit is your username for Steam. More info at: http://steampowered.com/