P3D is short for Prepar3D, which, as the two gentlemen have explained, is a flight simulator just like FS2004/FS9, FSX or X-Plane.
Versions 1, 2 and 3 of P3D are 32-bit, and many add-ons for FSX are compatible with those versions. But versions 4 and 5 are 64-bit, and that limits the backwards compatibility severely.
In addition, all versions are more demanding of the computer hardware than FSX, especially the later versions.
More information here: https://www.prepar3d.com/.
Plus, on this site there is a Prepar3D forum.
Jorgen