Do you write at the meeting or in the meeting here? Why?What is the difference in meaning btw them?

In very broad terms at the meeting / at a meeting shows physical location, and in the meeting / in a meeting shows mental location -- in the sense of informational content -- or temporal location -- in the sense of being occupied with an activity. at is more literal; in is more figurative.

-- I couldn't find Mr. Simpson this morning. Where was he?
-- He was at a meeting in Conference Room B.
(physical location)

-- Can I talk to Miss Jensen now?
-- No. She's on the phone. She's in a meeting with the regional sales coordinator.
(occupied with an activity)

-- Did you go to the all-employee meeting?
-- Yes. Very boring!
-- Who was at the meeting?
(physical presence)

-- What was discussed in the meeting?
(informational content of the activities of the meeting; duration, i.e., during the meeting)
-- Mostly the disadvantages of launching our new product before January.

That said, the two are often interchanged because it is sometimes unimportant to make the distinction.

-- What was discussed at the meeting? (physical location)

Mostly at. But:

at the meeting: there, at that meeting, will be discussing that subject

in the meeting: here, in this meeting (where we all participate in now) we're discussing this subject