"On top of that" is an idiomatic expression used to introduce an additional item, fact, or reason. It is commonly used in conversations to add further information or emphasize a point. It’s similar in meaning to "in addition," "besides," or "moreover."

For example:

The project was already behind schedule, and on top of that, we lost a key team member due to illness.

I had a busy day at work, and on top of that, I had to pick up groceries on my way home.

She’s very talented, and on top of that, she’s really hardworking.