What is a piece of glass in front of the cockpit of an airplane?

That's a head-up display, which shows some necessary data for flight without affecting the field of vision.

The eggshells in the wind are confusing you again. Who told you that the radar display is a head-up display? As far as the current layout is concerned, radar enemy search, mission map and combat are all on the lower left display, and the head-up display only shows the positive range and main targets. The main information displayed by the head-up display is altitude, speed, attitude, AOA, G value and various weapons auxiliary information.

Look at the picture: