"Accidentally" is an adverb that means something happens by chance or without intention. It's the opposite of "intentionally."
For example, if you accidentally spill your coffee on your shirt, it means you didn't mean to spill it, but it happened anyway.
Here are some more examples:
1. I accidentally hit the snooze button on my alarm clock and slept in.
2. She accidentally left her keys in the door when she locked it.
3. He accidentally backed into the neighbor's mailbox while parking his car.
