In all legal and even illegal poker games these days, table stakes play, which means that announcing all in simply means you are betting the chips you have on the table. Long gone are the days when a rich player could announce a bet of a million dollars out of the blue, and you would have to fold if you didn't have the money to cover that. This means you can never be asked to call for more money than you have on the table.
The most important of all poker rules related to all in is that poker is always played with table stakes. Moving all in simply means betting all your chips on the table, and it is simply another bet or raise in a given situation.
The idea of going all in can be a bit scary to new poker players, but that’s only because of the mystery that surrounds it, often created by TV poker.