Things with long wavelengths are analogous to the basketball in the cave 
story because neither can provide too much detail about what they hit. 
Things with short wavelengths are like the marbles in that 
they can provide you with fairly 
detailed information about what they hit. 
The shorter the probe's wavelength is, 
the more information you can get about the target.
A good example of the wavelength 
vs. resolution issue is a swimming pool. If you have a swimming pool 
with waves which are 1 meter apart (a 1 meter wavelength) and push a stick 
into the water, the pool's waves 
just pass around the stick because the 
1 meter wavelength means that the 
pool's waves won't be affected by such a tiny target. 
All particles have wave properties (how is that possible?) So, when using a particle as a probe, we need to use particles with short wavelengths to get detailed information about small things. As a rough rule of thumb, a particle can only probe down to distances equal to the particle's wavelength. To probe down to smaller scales, the probe's wavelength has to be made smaller.
Read more about waves if you are confused about the fundamental properties of waves, or if you don't understand how particles can behave like waves.