This is really not a Unity question - it’s a basic maths question. But I’ll try to help you out.
A Vector is a bunch of numbers. A Vector3, which is the most common sort you’ll be using in Unity, is 3 numbers, specifically. When used to describe a position, those numbers represent the distance from an origin in 3 dimensions: x, y, and z.
But if Vector3s confuse you, let’s start with a one-dimensional example instead: a line along the x-axis. So we can describe any position on a line with a single number, representing it’s distance from the origin. Let’s say I’m at position A (x=10), you’re at position B (x=6). How do I work out how to get from A to B?
Well, I need to take away 4, right? So it’s B - A, which is 6 - 10 = -4. It’s just the difference between them.
Now let’s do it in two dimensions. So A and B are now Vector2s. Say that A is (3,10) and B is (8,5). How do you get from A to B now?
So you need to add 5 to the x component of the vector, and take off 5 from the second component of the vector, right? To get from A to B is now (5, -5). Again, you work this out as B - A: (8,5) - (3,10) = (5,-5).
You can follow this approach for vectors of any higher orders as well. To work out how to get from A to B, you just do B - A for each of the components of the vector.
Try Vectors and spaces | Linear algebra | Math | Khan Academy if you’re still confused.