Definition
An invitation to a guest in the speaker's home to treat the house as if it were theirs.
How People Actually Use It
There are no hostilities toward you (singular or plural) here, treat this place as if it were your own. When used to unknown females, is often a smarmy indication that she will be molested.
"Come in, young gypsie girl traveller, make yourself at home. *rapes the young traveller later that night*"
Source: Urban Dictionary