“Stock” means use the developer as the stock solution, with no further dilution for processing—effectively 1+0.

In darkroom chemistry, the terms are usually:

• concentrate: the unopened liquid or powder form that must be mixed
• stock solution: the developer after you’ve mixed the concentrate or powder according to the manufacturer’s instructions
• working solution: the final dilution actually used to process film or paper

So if the chart says “stock,” first prepare the developer into its normal stock solution as directed by the manufacturer, then use that solution directly without adding extra water.

By comparison, entries like 1+1 or 1+3 mean take the stock solution and dilute it further with water before use.

A general rule is that concentrates and stock solutions usually keep longer than working solutions, which tend to have a shorter shelf life.