On a graph, each equation is represented as a line. The solution to simultaneous equations is the point where the two lines intersect, because that point lies on both lines.

A pair of simultaneous equations usually has two unknowns, such as $x$ and $y$. A solution is one pair of values that makes both equations true at the same time.

You can check a proposed solution by substituting it into each equation. If it works in only one equation, it is not the solution to the system.