These are fire stopping pillows. These, as you can see are used in fire walls/barriers for large openings.
They are typically used with large data trays or pass throughs that require a rated application.
The important thing to remember here is that there can only be spaces of a specific size. If the spaces left between the pillows and the trays and walls is too large, it will not seal up correctly when heated.
Basically, the way that these pillows work is that when they are heated, they expand (the plastic will melt away) and fill the void. This maintains the fire rating and prevents fire from spreading and smoke spread is contained.
There are several brands and each has it’s own specific way to apply it. Don’t forget, there must be an UL listing for each application and product to meet the minimum fire rating requirements. Make the contractor produce this and show you that it is appropriate.
Stay safe and be careful.