Three major conventions are used in modeling to represent the dynamics of an ecosystem. Pools represent where matter is stored, and we represent pools by the box icon. In Stella, pools are boxes. Here, the pool is alive plant biomass. In Stella, fluxes are arrows.
Parameters are usually rate constants that define how fast matter is moved into or out of pools.
In Stella, parameters are circles that are linked to the fluxes. Time step defines the interval of time in the model and is usually defined by the scale of your question. If you are interested in leaf CO2 uptake, you need a time step of seconds-hours. If you are modeling seasonal changes in leaf biomass, you need a weekly or monthly time step to capture those changes. If you are modeling changes in soil carbon, a time step of a year is fine.