DSL stands for Digital Subscriber Line, and is a type of technology that transmits data over a phone line.
This is very old equipment and mostly obsolete in the days of much faster fibre broadband.
The most commonly used type of DSL is ADSL (Asymmetric Digital Subscriber Line) typically known as standard broadband, which allows connection to the internet. ADSL works simultaneously with a telephone line so you can access the internet and still use it for calls, unlike the previous generation of internet technology, dial-up.
DSL’s days are numbered with the growing prominence of fibre broadband which allows for much greater data speeds. However, very cheap broadband using DSL services may still appeal to some people.