GStreamer: what mean 'preroll'¿

[dooteo <dooteo zundan com>]

Hi all,

In GStreamer POT file, there are message with 'preroll' word.

In Spanish and French languages, they translate as 'prepare'. But
looking in web sites

	http://www.gearwire.com/qa-cubasesx-recording.html#Q08

it looks that 'preroll' (and postroll) are special marks to select the
begin and end of a timeline, where is going to record a sound.

In other sites they explain (from video view) as:

http://www.jeroenwijering.com/?thread=6631

"That's the definition of a preroll, it rolls before the movie." 

and I guest it could works for sound too... Unfortunatly i don't
understand 'it rolls before the movie' expression.

Can somebody explain me what it stands for? Or where to found a document
to understand it?

Thanks and best regards,

Dooteo