。
She pinched my arm hard.
Literal
She [topic-は] my arm [object-を] tightly pinched.
つねる (also written 抓る) is the specific verb for pinching — applying pressure between thumb and finger. きつく is the adverbial form of きつい ('tight, severe, intense'), here intensifying the pinch. つねる is one of those everyday verbs that doesn't quite line up with English: where English splits 'pinch' into pinching with fingers vs. pinching with a tool, Japanese つねる is reserved for the finger-pinch. Standard in childhood-mischief contexts and slapstick.