SKILL Language is cadence specific languages that allow you program some custom command used in IC design. I think this is really related to the topic of the forum but we should put it in capital form.
The same question was just recently asked in the PC programming forum which is probably the more appropriate forum to ask this in unless Cadence also uses SKILL in their analog IC design tool set?
See the link:
A good book on LISP (on which SKILL is based) is probably the best way to get started. After that the Cadence SKILL documentation will probably make a lot more sense.