You’re fortunate if someone in your family tree is amongst the 59,221 name indexed in this collection. You get to see an image of the original one-page service file sourced from ADM 337 at The (UK) National Archives linked to the index.
These volunteers were clerks, bakers, labourers … not seafarers by trade, but they wanted to serve their country in the navy in the event of war.
MyHeritage has the same records, and a few more, 59,784.
Findmypast includes these in a more extensive British Royal Naval Reserve 1899-1930 collection which includes those in ADM 240 as well as ADM 337 for a total 81,544 records.
The records are handwritten and include abbreviations for which there is no guide, even that at TNA.