SID are a coded departure route established to expedite departures at airports with lage volume of traffic.
They are intended to simplify clearance delivery and departure procedures for both you as a pilot and ATC air traffic control.
When you are operating from an airport where SIDs are published, you can expect to have SID routing included in your ATC clearance.
You dont have to accept a SID, but you must advise ATC if you do not wish to use one.
You need to mark NO SID in the remarks section of your flight plan.
To fly a SID, you must have the charted procedure or at least the textual description, otherwise you need to file NO SID in your flight plan.