First and most importantly, (sentence)
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.