。
She tied her shoelaces.
Literal
She [topic-は] shoes' string [object-を] tightened.
靴のひも is literally 'shoe's string' — Japanese constructs 'shoelace' as a possessive compound rather than as a single lexicalized word. ひも is the general word for any kind of cord, string, or strap. 締める is the verb for tightening or fastening — used for tying laces, fastening belts, tightening screws, even closing a deal (商談を締める). It's the opposite of 緩める ('to loosen').