Someone or something that is mysterious is strange and is not known about or understood.
He died in mysterious circumstances.
他死得蹊跷。
A mysterious illness confined him to bed for over a month.
一种怪病让他卧床一个多月。
If someone is mysterious about something, they deliberately do not talk much about it, sometimes because they want to make people more interested in it.
As for his job – well, he was very mysterious about it.
至于他的工作–嘿,他对此十分诡秘。