Hi Zoe,
The relationship between "axis" and "axes" depends on the context.
An "axis" can be an imaginary line which an object rotates around. Example: The tilt of the Earth's axis gives us our predictable seasons.
When we draw graphs to represent our experimental data, we need two "axes" -- a vertical "axis" and a horizontal "axis". Usually, the horizontal "axis" depicts the independent variable while the vertical "axis" represents the dependent variable. In this context, "axes" is the plural noun form of "axis".
When we want to chop some firewood, we use an "axe" -- a tool with a sharp blade to fell a tree. A group of lumberjacks can gather to fell many trees with their "axes", which is the plural noun form of "axe".
Just as a side discussion, an "axis" can also refer to an alliance among some countries. Example: "An axis of evil "
Hope this helps.
Cheers,
Lance