Stocks and flows are two different types of variables that play important roles in economic models. In fact, you will almost certainly never find a model without at least one of these two types of variables, no matter how far you advance in your economics education. Obviously, this is an important concept to understand.
Both stock and flow variables are defined in reference to time. Being embedded in the fabric of time, being able to discern values in different time periods past, present, and future will be a substantial boon to learning about how the world works in any discipline, but especially in deciphering the constantly-changing economies of the world.
Stock Variable- A stock variable is represents the value of something that is measurable at a single point in time. For instance, wealth, inventory, and number of vehicles per household are examples. A stock variable is kind of like a still camera picture, it represents a quantity that can be measured at a point in time. In other words, it has no time dimension.
Flow Variable- A flow variable represent the value of something that can only be measured in reference to time. GDP, personal income, and consumption are examples. A flow variable is like a video, it represents something that must be measured in reference to tie. In other words, it does have a time dimension.
For example, GDP over the course of the last month and GDP over the last years are two different flow variables. Wealth one month ago, and wealth one year ago are two different stock variables. While stock variables can change throughout time, they are not related to time in the same way that flow variables are.