I think that it stands for Trinity Broadcasting Network.
How Christian is truly is will become a matter of debate on here.
For instance...
http://www.pfo.org/tbnboot.htm
http://www.apologeticsindex.org/t27.html
http://www.letusreason.org/Pent49.htm
http://www.ondoctrine.com/10tbn.htm
I've never watched but have read descriptions of a lot of their shows... at best it's greeting card Christianity, not the real deal.
Should be "it truly is"