Flows are defined over a period of time.
Stocks are defined at a particular point of time.
To understand the difference between stock variables and flow variables, let us take the following example. Suppose a tank is being filled with water coming from a tap. The amount of water which is flowing into the tank from the tap per minute is a flow. But how much water there is in the tank at a particular point of time is a stock concept.
However we can measure a change in stock over a specific period of time like how many machines were added this year. Such changes in stocks are thus flows, which can be measured over specific time periods. For instance, A particular machine can be part of the capital stock for many years (unless it wears out); but that machine can be part of the flow of new machines added to the capital stock only for a single year when it was initially installed.