This is the 2479th most frequent Japanese word.
兄さん
Older brother.
In this sentence, '兄さん' is used as a term to address one's own older brother in a casual but respectful manner.
兄さんは親切に手伝ってくれました。
My older brother kindly helped me.
'兄さん' here refers to an older brother in a general sense, not necessarily specific to the speaker's family.
その少年が兄さんに似ているとみんなが言っています。
Everyone says that boy resembles an older brother.
In this case, '兄さん' is used with 'お' as an honorific prefix to politely refer to someone else's older brother.