There is not much to explain, but here are a few ways to use the number of employees in your research. First, if you are comparing companies within an industry, you can find ones that have the same number of employees to help ensure you are comparing apples to apples.
Secondly, using the number of employees can help gauge how the company is growing. If the company is not hiring more people, it could mean they are stagnant or losing market share to competition. Now it is important to note if management has stated they are no long expanding, then you do not want to see that number grow by large numbers each year.
Lastly, this could be a negative indicator if the company is going through a rough period and people are being laid off or fired. You want to see the number move in accordance to what the company believes is going to happen in the future. There will be rough points, but as long as the company is optimistic, do not look into the change numbers of employees too deeply.
This number is simple and usually over looked, but can be a great little glimpse into the health of the economy. Use it to compare the right size of companies, that way you are in the right area and ballpark, and comparing accurate data points. If you have ideas, bounce them off people in the investment community, getting real time feedback and adjustments.