Heya mate,
If you're reffering to the Diamond or Pearl server, Skill Credits are used for your /skills which can be more explained at /mmo info. The amount of credits can be viewed at /credits and redeemed by doing /redeemcredits <skill> <amount>. To see what perks it gives to level them you can do /jobs and shift-click each individual to see how high of a level you need to unluck perks like having more Ranches, /autosell, access to Thunderstone armor, and some more things!
Hope this helps,