For instance if you have a triangle in the upper right quadrant of an x,y coordinate grid and you want to reflect it over the y-axis you would take each point of the triangle and move them to the other side of the axis. If one point of the triangle was 3 units to the right of the axis, then the new point would be 3 units to the left of the axis.

If you are doing a translation of an image then each point is moved right or left or up or down.

Directions for translations are usually given like this, (x,y) goes to(x +4)(y + 2). That would mean move the points of the original image over 4 to the right and 2 up.

Hope this helps, give it a try on some graph paper.