Parameters are powerful concepts within Stringify. They are essentially variables whose values get populated when the flow runs. They are contextual, meaning you'll only see parameters that are specific to the things in a flow.
Image: Example of flow parameter
Different things have different parameters - values that are specific to the that thing.
Generally speaking, when configuring a thing, if that thing supports parameters, a + symbol will be shown next to the appropriate field. Simply tap on the + symbol to bring up the list of available parameters for this thing. Tap on the parameters desired, then tap Save or Done.