The "profit margin" for a business is the amount of money that the business keeps after paying its expenses.
For example, a grocery store might buy a box of cookies for $1.00 and sell it for $2.00. In that case, the "profit margin" is 50%.
Here are a few phrases that people use when talking about "profit margins":
Our profit margins are slim.
They have huge profit margins.
You have to have a healthy profit margin.