What are the BUGS and RST buttons on the A320's ISIS for?
The RST button is the attitude reset (note the ATT next to the button) function. The ISIS contains an attitude gyro that has to be re-aligned periodically.   


Credit: Smartcockpit.com  

The BUGS button enables the BUGS function, which allows the crew to enter speed and altitude bugs on the display. The bugs are a visual identifier to aid the crew in maintaining a speed or altitude.  



Credit: Smartcockpit.com