A flow is simply the rate of change of a stock. Returning to our bathtub analogy, the rate of inflow to the tub would be the flow of water coming from the tap/faucet. The rate of outflow from the tub would be the flow of water exiting via the plughold.
Whether the bathtub fills up, or drains, is simply the net effect of the in- and outflows. In other words, when the rate of inflow exceeds the rate of outflow, the tub fills up - and vice versa.
Many problems in business simulation are analogous - for example customers acquired are an inflow to the stock of existing customers. Customers leaving (sometimes called churning), would be an outflow from the stock of existing customers.
You can add a flow to a sheetless model by pressing the 'f' key.