If something happens ominously, it happens in a way that makes you feel like something bad might happen soon.
The word ominously makes people feel worried or scared.
Examples:
The dark clouds moved ominously across the sky.
The old house creaked ominously in the night.
The teacher looked at the students ominously.
The cat stared at the mouse ominously.
The ground shook ominously, and everyone ran away.
The music changed ominously, and the audience felt scared.