もの originally means a thing.
However, as Amano-san wrote, it has many meanings and usages.
So, I will explain some of them.

〇ものだ／ものです
・express accepted thinking with strong emotion.

赤ん坊は泣くものだ。
あかんぼうは　なくものだ。
Babies are meant to cry.

・express strong feelings or admiration, often when comparing something between now and before.

この町も便利になったものだ。
このまちも　べんりに　なったものだ。
This town has gotten convenient now.

〇もの／もん（more casual)
describes a reason, often used to insist on the legitimacy.
Often used by young women or children.
Often used with だって by children when they talk back.

どうして人参をたべないの？
どうして　にんじんを　たべないの？
Why don't you eat a carrot?
だって、人参おいしくないもん。
だって、にんじん　おいしくなもん。
Because a carrot is not delicious.

また東京に行くの？
また　とうきょうに　いくの？
Do you go to Tokyo again?
お友達がスカイツリーに行きたいと言うのだもの。
おともだちが　すかいつりーに　いきたいと　いうのだもの。
Because my friend says she wants to go to the Sky Tree.

I hopes this helps you a bit.