Docker is a system for running multiple operating systems on one computer. It's like a virtual machine, but more flexible, because the different instances share one kernel.
YOUTUBE eGz9DS-aIeY Here's a good intro. Network Chuck has a lot of…personality, but there's good info here.
# See also
YOUTUBE Qw9zlE3t8Ko Here's a very fast, rather advanced intro to docker-compose. It's very well thought out, worth watching even if it's a bit over your head. Some good insights into how docker "thinks".
YOUTUBE MVIcrmeV_6c Another good, short intro to docker-compose