This is the 402nd most frequent Japanese word.
社
"社" (sha) means "shrine," "company," or "society," depending on context.
In this sentence, '社' refers to a company, used as part of '会社' (kaisha) meaning 'company.'
この会社で働いています。
I work at this company.
Here, '社' refers to a shrine, as seen in '神社' (jinja), which means a Shinto shrine and is a place of worship.
あの神社は有名です。
That shrine is famous.
In this context, '社' is used in '新聞社' (shinbunsha) to denote a company specializing in newspapers, and '入社' (nyūsha) means joining a company.