This is the 3033rd most frequent Russian word.
заметить
To notice.
In this sentence, "заметить" means "to notice" or "to take note of" in the context of identifying an error.
Я заметил ошибку в документе.
I noticed a mistake in the document.
Here, "заметить" is used to mean "to perceive" or "to become aware of," applied to realizing someone's action.
Она заметила, что кто-то наблюдает за ней.
She noticed that someone was watching her.
In this instance, "заметить" indicates "to observe" or "to take mind of," referring to unusual conduct.