Could somebody explain in details what are all these mean?
So much is going on...

And why so much bar is shown when we need tops 3 bar or 50 psi

Especially interested in that blue r134a line, what is it mean and how to use it?

 

Here is a chart for one of my cars, along with an explanation of readings.

 

Nooblex, this chart gives a "ballpark range" for what you should be seeing for pressures.

What you are seeing on the gauges:
1) Outside ring is showing pressure in BARS
2) Next ring is ambient air temperature in degrees C when using R134 refrigerant. At rest(engine off)when the low side and high side pressures balance out(eventually), both needles should lay approximately on the current outside air temp shown on your gauges in degrees C. Not perfect but a way to see if your refrigerant charge is close. But, I have found these temp rings not accurate enough for best ac performance- it's just a guide. Your low pressure and high pressure gauges should in theory read the same pressure when engine is off and some time passes and the next ring should be close to the current outside air temperature.

How to read Esullys chart: AC compressor must be running.

Example: Ok, using the chart you can see if your outside ambient air temperature is 21C your high side pressure should be in the range of 10-11 BAR(145-160 PSI) and your low side pressure should be in the range of 2.4-2.8 BAR(35-40 PSI).

*Note- the chart is just a guide, there are tricks and experience needed to get max cooling but the chart will let you know if problems exist.
*note- see the other chart for possible solutions if pressures are too high or loo tow.