。
She held the baby tightly in her arms.
Literal
She [topic-は] baby [object-を] firmly arms [in-に] held-tight.
しっかりと means 'firmly' or 'tightly' — しっかり is a 擬態語 (state mimetic) evoking firmness, solidity, or resolve, and と turns it into an adverb. Combined with 抱きしめる (to hold tight), the double emphasis creates a vivid image of a protective, secure embrace.