What does this bar mean?

The white bar on the left

That is the engine evaluation of the current position. + is in favour of white, - is in favour of black. Based on the values attributed to the pieces 1 for a pawn, 3 for knight and bishop, 5 for a rook, 9 for a queen. In the position you present its +5.80 so as though white is up a rook and a pawn (+6). As you see, white is actually only up 2 pawns (+2). The extra + is because white also has a massive positional advantage. 

Could be +5.80 is the length in kilometres of his escape tunnel so far...

lol

It’s basically the positional advantage in a game.

Not true. It can be material or whatever else kind of advantage as well

how do i turn this bar on? i'm not getting this bar.

During a game, you can only turn the bar on when playing against bots

And I thought "+5.80" is the actual number of pawns captured.

Four-fifths of a pawn captured.

how do i turn this bar on?

It's only available after games, in analysis, while spectating, or during bot games in certain modes.

While analyzing, you will see this toggle in the upper-right corner:



If Evaluation is enabled, you will see the bar. You will also see that same toggle when choosing custom settings for games against bots.

What is the -M1 notation in the bar ???

Black has mate in 1

M and a # means mate in that number of moves. So M1 means mate in one move.