形式名詞N4
〜もの（形式名詞）
thing (nominalizer)
Structure
V/い形容詞（普通形） + もの；N + の + もの
Explanation
As a formal noun, もの refers to concrete things or objects, equivalent to "the thing that...". It can also express abstract matters. E.g., 買ったもの (what I bought).
Examples
これは昨日買ったものです。
This is what I bought yesterday.
食べたいものを選んでください。
Please choose what you want to eat.
Note
In casual speech もの is often pronounced もん, adding a playful or defensive tone. Note the difference from 〜ものだ (ought to / a general truth).