Expresses that once a certain action happens, a situation changes irreversibly; 'once', 'after'.
Long explanation:
The たら最後 grammar point is used to convey that once a certain action takes place, the situation changes in an irreversible way, usually for the worse. In English, this could be translated as 'once' or 'after'. To form たら最後, conjugate the verb to た-form (past tense) and attach '最後'.