What does it mean when a transaction stucks?
It means it hasn't received a confirmation.
That means its not realible?
Correct, those transactions shouldn't be trusted.
If it stucks then you might lose money?
Indeed. The other involved party could "double spend" it or the transaction could be dropped from the network (low fee) and returned to the originated address (as result, you'll lose everything). It's best to wait for multiple confirmations (3+) before trusting it.