Whether we call it Self-driving car, Autonomous Vehicle (AV) or Driver Less Auto, the technology is meant to save lives, reduce human error, decrease the number of accidents while efficiently moving people or goods using the transport system infrastructure.
United State Department of Transport U.S. D.O.T defines 5 levels of vehicle automation:

Level 0 being a Vehicle with Driver Warning System capability, it is supposed to provide guidance to the driver, it makes no decisions and takes no control. Features for Level 0 would range from blind spot monitoring, forward collision warning to lane departure warning

Level 1 being a Vehicle where Isolated Driver Functions are isolated, it requires human driver to perform essential functions while it manages individual driver functions in an automated manner. Features for Level 1 vary between adaptive cruise control, dynamic brake support to lane keeping assist functions

Level 2 supports automation of multiple driver functions, but still requires a human driver to handle essential functions. It can simultaneously manage adaptive cruise control with dynamic brake support and lane keeping assist

Level 3 defines a vehicle with limited Self Driving capability, it handles limited autonomous operations in certain environments with supervision from human driver to handle complex situations. Functions in level 3 react properly to pedestrians, handle railroad crossings, interpret traffic signals and respond to inclement weather

What is autonomous mode?

“Autonomous mode” is the status of vehicle operation where technology that is a combination of hardware and software, remote and/or on-board, performs the dynamic driving task, with or without a natural person actively supervising the autonomous technology’s performance of the dynamic driving task. An autonomous vehicle is operating or driving in autonomous mode when it is operated or driven with the autonomous technology engaged.
Level 4 is a fully autonomous vehicle, capable of handling advanced driving situations and environments, from origin to destination. Interpreting and avoiding animals and obstacles, obeying instructions from flaggers and reacting properly to bicyclists are few functions to mention.