This idiom means to start a process or situation that cannot be easily stopped or reversed. It's based on the story of Aladdin and the genie, where the genie grants wishes but cannot be put back in the bottle once released.
Here are some examples of how it's used:
* "Once the rumors about the scandal started to circulate, the company knew they'd let the genie out of the bottle and couldn't control the damage."
* "By revealing the secret formula, they let the genie out of the bottle, and now competitors can easily copy their product."
In essence, it signifies irreversible consequences or unforeseen consequences of an action.