If you are using your RaspberryPi with display, e.g., like here, you probably want to launch it to the kiosk mode after start or reboot automatically.
The simplest way is to execute a script. In the script you can start Chromium or Epiphany browser, etc. The following code snippet represents a file start_kiosk.sh. It launches Epiphany browser on localhost in full-screen mode.
epiphany-browser -a --profile /home/pi/.config http://localhost --display=:0 & sleep 15s xte "key F11" -x:0
The next step is to setup script execution. It can be done by adding execution script into file /home/pi/.config/lxsession/LXDE-pi/autostart:
By this setup you can change your Raspberry into a simple kiosk device. There exist more sophisticated solution for the real kiosk mode, e.g., disabling access to users, etc.
This simple solution just save manual start of the browser or other app after Raspberry start.