Let me give you an example to illustrate the difference.
Imagine there’s a business meeting. There are 10 people when the meeting begins. You happen to pass by the meeting room. How many people do you see?
You see 10 people.
Before the meeting is over, 3 people leave the meeting for some reason. You happen to pass by the meeting room again. How many people do you see now?
You see 7 people.
In terms of grammar this means two things:
There are seven people left.
Three people have left.
There are seven people left because three people have left.
“are left” means “stay, remain”
In this example it refers to the people who have stayed in the meeting. In other words, the people who didn’t go away.
These are the people who are still present in the meeting.
“have left” means “gone away”
Here it refers to the people who did not stay in the meeting. In other words, the people who have gone away. These are the people who are not present in the meeting now.
Either way, the math is the same. Two different ways of saying the same thing.
It’s simple, but it’s something learners often mix up. Hope this explanation clarifies the difference.