At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
Usually, you have a single equation and a single variable (x+5=10). The only constraints we have on that variable come from that single equation.
Sometimes, however, we have a more complicated system with multiple variables, but we also know more about it. In that case, we have 2 equations, each of which describe constraints about 2 variables and how they relate to each other. Think of them as existing at the same time (hence the name simultaneous). Then we solve both of them together to find both variables.
If you're a graphical person, think of each equation as a line in space. When we have one variable, we're working in 1-dimensional space (a line), and the equation tells us where on the number line our answer is. When we have two variables, we're working in 2-dimensional space (a plane), and each equation is a line. Where they intersect is the ordered solution pair. You can obviously take this concept up to 3 or more variables, in 3-space, 4-space, etc.