A SID is a standard instrument departure. A STAR is a standard terminal arrival route. They're available as diagramatic interpretations, and used by aircrafts to land and take-off from airports and airfields. It's essentially a sequence of movements.
Yeah... So like Jacob mentioned, STARs and SIDs are the set routes to approach and depart an airfield.
So this for example, is an expired chart, showing the BRI1A,B,C & E arrival routes to Bristol airport. So for example, following the BRI 1E arrival route, the first VOR is TALGA, to BRECON and then to BRISTOL airport. [ie. arriving from Dublin western parts of Ireland, TALGA is likely the VOR that you'd have already on your route]
I don't know if I'm teaching you to suck eggs, but if this is all new information, then I suggest watching a few YouTube videos to further clarify, there are loads to choose from.
Usually, I use NAVIGRAPH for my approach and departure charts, which are a little clearer on altitude constraints, but for here you can see at AMRAL you must be at 16,000 feet, and AMRAL is 10 nm from RILES.
I'm not an Xplane user but I do have many hours in flight simulators. What @SeaHawk14 described above is accurate, SIDs and STARs are set routes to get in and out of busier airfields to keep the neighbours happy about noise and stop traffic conflicting. They would typically join up with the rest of your flight plan route.
Current UK charts can be found at NATS | AIS - Home the SIDs are named after where they finish, but annoyingly UK STARs are named after where they finish too so it's not immediately obvious which STAR would join up with your flight plan. Also in the UK you have standard routing to comply with too.