This is the 500th most frequent Hebrew word.
עצמם
"Themselves."
In this sentence, 'עצמם' (themselves) is a reflexive pronoun referring back to 'הם' (they), denoting an action they directed towards themselves.
הם ראו את עצמם במראה.
They saw themselves in the mirror.
Here, 'עצמם' (themselves) is used to emphasize self-assurance within the context of their feelings.
הם רוצים להיות בטוחים בעצמם.
They want to feel confident in themselves.
In this context, 'עצמם' (themselves) is employed to indicate that they performed the action independently, without outside help.