(For best understanding, see the song "I'm A Fake" by The Used.)
1. The act of pretending to be something (i.e. happy, normal, sane...) in order to keep everyone from thinking you are something other then that (i.e. sad, weird, crazy...).
People like this are usually attempting to protect themselves from the harsh reality of whatever is around them. Sometimes, rarely, they are trying to live up to the reputation that may have been put on them years ago.
2: This word can have whatever meaning you want it to. It is defined by the person using it. More of a personal understanding.
(See the song "I'm A Fake")
"She always seemed so happy."
"Heh. She was fakecore."
"The newest trend was people claiming they where fakecore, however none of them knew what it actually meant. But it did sum them all up rather well."