This is the 1593rd most frequent Portuguese word.
sério
Serious.
Here, 'sério' is used as an adjective to describe the nature of what was said, meaning important or grave.
Ele falou algo muito sério durante a reunião.
He said something very serious during the meeting.
In this case, 'sério' is also an adjective but used to confirm whether someone is not joking or being genuine.
Você está falando sério mesmo?
Are you really being serious?
Here, 'sério' characterizes the nature of the proposal, implying a strong commitment or thoughtfulness behind it.