。
She knocked on the door.
Literal
She [topic-は] door [object-を] knock did.
ノックする is a productive loanword-verb construction: take a noun (here ノック, a loanword from English 'knock') and append する to verb it. This する-attachment is one of the most prolific patterns in modern Japanese for absorbing new vocabulary — チェックする ('check'), キャンセルする ('cancel'), ググる (irregular: 'to google,' with the ru-verb shortcut). The native verb for knocking is 叩く ('strike, tap'), but ノックする has settled in as the default for politely tapping at a door — Japanese homes traditionally have sliding doors (襖, 障子), which you don't really knock on, so the loanword filled a vocabulary gap as Western-style doors came into use.