Think of all the rectangles whose perimeters are 8 units. If $x$ represents the width and $y$ represents the length, then $$2x+2y=8$$ expresses the relationship between the width and length for all such rectangles.
For example, the width and length could be 1 and 3, since $2 \boldcdot 1 + 2 \boldcdot 3 = 8$ or the width and length could be 2.75 and 1.25, since $2 \boldcdot (2.75) + 2 \boldcdot (1.25) = 8$.
We could find many other possible pairs of width and length, $(x,y)$, that make the equation true—that is, pairs $(x,y)$ that when substituted into the equation make the left side and the right side equal.
A solution to an equation with two variables is any pair of values $(x,y)$ that make the equation true.
We can think of the pairs of numbers that are solutions of an equation as points on the coordinate plane. Here is a line created by all the points $(x,y)$ that are solutions to $2x+2y=8$. Every point on the line represents a rectangle whose perimeter is 8 units. All points not on the line are not solutions to $2x+2y=8$.