> What does it mean?
> Google shows, there also Run First can exist, however, it didn't bring me
> any links to the documenation.
Signals are not GStreamer specific thing, they are part of Glib's
GObject, which offers C++-like object implementation in pure C. You
can read more about GObject itself on GNOME's library site. Signal
flags that you're asking about are described in section about GObject
messaging systems and in API reference.
I would advice you to read the whole GObject documentation (apart from
API docs). GObject is an essential part of GStreamer and knowing some
details about it will make it easier to code.