You get it by choosing it. There is no further prerequisite except what the DM adds. You should probably have a backstory of where the heirloom comes from (grandfather, father, teacher, whatever), but that's less "written rules" and more "common sense."
Oh, re-reading you don't know what traits are. In the Advanced Player's Guide under New Rules there is a section called "traits."
At character creation characters are allowed to get a number (usually two, but DM discretion) of these traits, which usually offer some tiny and heavily restricted bonus, like a +1 to a save or a single skill as a class skill. These traits tie into the background of a character and are just as often about "flavor" as "power."
The Ultimate Campaign Guide further offered "drawbacks" which would allow a character to take a specific weakness in order to gain another trait.
The traits and drawbacks range as widely as feats, from the useful to the terrible, and are sometimes nice simply because they are fun. My Cleric of Apsu has one that gives a +2 on diplomacy with dragons. That will never, ever be useful but darned if I didn't take it because I'm unhealthily obsessed with dragons.
