Some soft-wares are provided in .appimage format.
By default settings on your mint or ubuntu linux, you cannot execute it as a program.
For eg: the software Chiaki for remote play PS4 games on your Linux.
Copy the software to your home folder (main folder with your name).
Open terminal (Ctrl + Alt + T)
chmod +x <file>.AppImage
(Where <file> is your file name)
chmod +x Chiaki-v1.0.3-Linux-x86_64.AppImage.AppImage