。
She gripped my arm firmly.
Literal
She [topic-は] my arm [object-を] firmly grabbed.
しっかり is a state mimetic (擬態語) for firmness, tightness, solidity, or steadfastness. With つかむ, it specifies that the grab is solid and unyielding rather than loose. しっかり has a wide range of figurative uses too — しっかり勉強する ('study diligently'), しっかりしている ('be reliable, have one's act together'), しっかりした人 ('a steady, dependable person'). One of the most versatile and frequently encountered mimetics in everyday Japanese.