well, that's a very vague/general question. basically a line of symmetry of a function is any line that can be drawn where the function looks identical on both sides, that is, the line of symmetry acts as a mirror where we can reflect one side of the function to get the other side. for example, the y-axis is a line of symmetry for the function , since looks the same on both sides. if we fold the xy-plane at the y-axis, one side of the function will exactly overlap the other. see here and here for more info