We should know what the four lettes (D-A-L-I) stand for before we know what DALI is. 

D - Dagital

A - Addressable

L - Lighting

I - Interface

DALI is an interface for dagital addressable to lighting. It is a 2-way communications protocol that is used to provide control over, and communication between the components in a lighting system. DALI defines the commands that LED Drivers and ballasts need to recognize. So the system allows individual LED drivers and ballasts to building the conversation between user and DALIcontrollers, software in computers or building management systems. For an example of the conversion, ligthing system send a command 'down 50% ' to lighting 'A', the DALi driver or ballast of 'A ' got this command, and make down 50% of the lighting, and then feedback the result to lighting system. 

DALI helps make the light or a group light have a fixed addressable, so it can be easily controled by lighting system, improved too much management effective. 

DALI originated in the late 1990s and was defined in IEC60929. Since then it has been substantially modified. The current (2020) version of DALI is called DALI-2 and this is defined in IEC62386.

There is a very wide range of DALI enabled lighting control products available from all the leading manufacturers and it is now widely recognised to be the global standard for lighting control.