It implies that the person is trying to be in charge and give orders, even when it's not appropriate. It can also suggest that they are arrogant and self-important.
The term is usually used in a negative or critical way, often directed towards women.
Here are some synonyms for "bossy boots":
* Controlling
* Domineering
* Authoritarian
* Pushy
* Overbearing
* Dictatorial
* High-handed
Example:
"She's such a bossy boots. She thinks she can tell everyone what to do."