Bash completion or tab completion is the capability of the shell (bash in this case) to be able to autocomplete words for commands, parameters, etc. on hitting the tab key.
Bash completion for Gstreamer is the capability for gst-launch and gst-inspect to autocomplete elements and their
corresponding pads and parameters. For example, gst-inspect-1.0 v4l2<tab>
will show all the list of available elements
(plugins) in the systems that start with v4l2.