What do the numbers on the analysis page mean?

Hi, I'm trying to figure out how to use the analysis function. In the screenshot I'm on the "explore"-tab of a game analysis and it is blacks turn. On the right sight there are possible moves to play. But for example on the move a6 there is "+0.51" written. What exactly does that mean? That black gets a +0.51 advantage if he plays this move?
And why are all numbers positive?
And what do the percentage numbers mean? If black plays a6 there is only a 26% win rate for black?

That's is the position results from the Master games database, showing the win/draw/loss from the games in the database.from the next position described by the "moves"

It should be noted those may not be the actual moves but the pieces will be in the given position.

The numbers at the front are the engine evaluation for the position after the "move." A + value is a white advantage and the color of the background is white for that reason.

But why is every move from black an advantage for white?

White is considered to have an opening advantage and generally speaking, masters don't play lines that give away much of that advantage in the opening.

Ok thank you, but I have one last question: If I put the black queen on e7 here, it should be +1.07 for white but the eval bar only goes from 0.1 to 0.5. Why is that?