A pre-roll is list of specific items that can be obtained as drops that the game will roll to give you before the main drop table. And if these items aren’t rolled, then the main drop table is rolled.
In OSRS there are a bunch of drops that can be obtained from monsters and from certain activities (like from clue scrolls). Most monsters and events with a unique drop have pre-rolls.
For example, unique drops in the Chambers of Xeric include the Twisted Bow, Kodia Insignia, and Dexterous Prayer Scroll. And there are also main items that can be obtained too, such as Pure Essene, Lizardman Fangs, and Various runes.
When you finish the raid, the game will “pre roll” all of the unique items first. If none of these items come up, then the main drop table will be rolled.
And in the case where you get a pre-roll, you will not get a main item.
How Do Drop Rates Work?
All items, including pre-rolls, have a drop rate in OSRS.
This determines how likely you are to receive a particular drop from an activity.
An example of a pre-roll would be a Bandos Chestplate, which drops at a rate of 1/381. However, this drop is on a pre-determined drop table of Bandos armour pieces – which include the tassets and boots.
All together, these have a combined drop rate of 1/127.
What this basically means is that for every single Bandos you kill, you’ll have a 1/381 chance of obtaining that specific item.
