There was an earthquake while you were at work. Your office building is messed up and everyone is confused about what to do now. You think the most important thing is to find out if anyone is injured or needs help. You say:
Our top priority right now is making sure that everyone is OK.
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.
Your "top priority" is the thing that is most important to you. It's possible for your "top priority" to change depending on what is important at that time. If something needs to be done quickly, that can become the top priority until it's done:
Right now my top priority is getting this report finished.
It may be a little confusing, but we also use the phrase "highest priority". This phrase is used slightly differently. You use the phrase "highest priority" to talk about the thing that is most important to you in a more lng-term way:
Making sure that our customers are completely satisfied is our highest priority.
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