A dilation is a transformation that reduces or increases the dimensions of a figure while maintaining the same shape. It's kind of like zooming in and out on the shape.
In the image attached, you can see 2 squares the both have different dimensions. Notice that the figure on the left is a square with sides the each have a length of 1 unit. On the other hand, the figure on the right is a square with sides that have a length of 4 units. Since both of the figures have the same shape, this would represent a dilation.