。
She shyly glanced at him.
Literal
She [topic-は] embarrassed-looking [manner-に] him [object-を] briefly looked-at.
恥ずかしそうに uses ~そうに as an adverb form of ~そう ('appears to be') — 'looking embarrassed / shyly.' ちらっと is a 擬態語 for a brief, fleeting glance — quick and furtive. The combination paints a vivid picture: a shy, stolen glance. Two mimetic-like adverbs (~そうに and ちらっと) work together to convey manner and speed.