plug-in

Question

what is plug-in?
What is meant by this sentence:
web browser may rely on plug-ins to display multimedia content; such programs are launched by and interact with the web browser through kernel services.

Answer 1

To put simply, 'Extra software necessary to serve your request'

Like, Flash was necessary to play multimedia content, and JAVA was necessary to run servlets.

Nowadays they're barely needed at all.