The word simultaneous
means at the same time. When a problem involves two variables and
they are related by two equations, then the equations have to be
solved together, simultaneously.

For example, suppose you need to
find the values of x and y so that

2x + y = 7 and x - y = 8

Adding the two
equations together gives 3x = 15, from which x = 5, Substituting
x = 5 in the first equation gives y = -3. So, the solution satisfying
both equation is (x, y) = (5, -3). This solution corresponds to the
coordinates of the point of intersection of the two lines representing
the equations.