“First and most importantly, you need to make sure that all your students are accounted for.”
You're an administrator at a school. You're giving a presentation to the teachers about your school's plan for what to do in an emergency. You start by telling this to them.
First and most importantly, you need to make sure that all your students are accounted for.
People use words like "first", "second", "third", "last", and "finally" in presentations to talk about items in a list, or steps in a process:
First, it's extremely easy to use. Second, it's 30% less expensive than the leading competitor. And finally, our customer service is the best in the industry.
But the first item in a list is often special. So we combine "first" with some other word to introduce the first item. In the example above, the speaker uses the phrase "first and most importantly". Another phrase with a very similar meaning is "first and foremost":
First and foremost, it's extremely easy to use.
When something "is accounted for", it means that you know where it is. You usually use this phrase to talk about a group of things, like all of the students in your class:
Everyone's accounted for.
This means that you know where everyone is. They might not all be with you, but you know that, for example, 28 of them are in your class, 1 is at home sick, and 1 is in the school counselor's office.
Either people or things can be accounted for:
Before you leave, please make sure that all your valuables are accounted for.
The opposite of "accounted for" is "unaccounted for":
Two soldier have been killed, and three more are unaccounted for.
The phrase "make sure ___ is accounted for" is common, so you might want to memorize both parts of it together.
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.