To "make sure" means to check something again, so that you know that it's OK. When you want something to happen and it's important, you check to "make sure" that it happens. For example, before your house guests leave, you can tell them:
Make sure you've got everything.
Or when someone is grilling some meat:
A more formal version of this phrase is "make sure that (clause)":
Make sure that the pork chops are cooked all the way through.
In a corporate office job, people are very careful not to make any mistakes, so they often talk about "making sure" of things.