How to create a taxonomy terms "skills" with level "low", "medium", "high"
Without knowing how you're planning to use this in a larger context, here's what I would do:


Create a vocabulary (or possibly just a plain text field with a select list) for "skill"
Create a second field with a dropdown list for skill level (I probably wouldn't use a taxonomy for this).
Make the second field dependent on the first, per this Stackoverflow question about fieldsets and conditions