This is the 675th most frequent Japanese word.
佐々木
"佐々木" (Sasaki) is a Japanese surname.
Here, '佐々木' is used as a proper noun referring to someone's last name, followed by the polite suffix 'さん'.
佐々木さんは日本からのお客様です。
Mr./Ms. Sasaki is a guest from Japan.
In this sentence, '佐々木' is used as a proper noun referring to an individual, without an honorific suffix.
昨日のミーティングで、佐々木がアイデアを出しました。
During yesterday's meeting, Sasaki presented an idea.
'佐々木' here refers to a common family name, indicating a general reference to individuals with this name.