Answer should be in the order itself. Generally when a decision is "set aside" it means that the the prior decision for which an appeal was taken ( apparently your grant of unemployment) is no longer valid and has been overuled or reversed. The use of the term may be due to the wording in the unemployment statute in your state. Have a lawyer in your state review the decision ASAP, if it has been reversed and you want to appeal there are time limits for that if you miss them you may be giving up your right to appeal if the decision is against you.
Legal disclaimer: This response is meant to be information only and should not be considered to be legal advice. This information is not meant and should not be construed to be the formation of an attorney client relationship. Employment laws may vary by state and you should contact an attorney in your state to see if a state law may be applicable to your situation.
Set aside does mean that the decision was reversed as the above commenter posted. If you would like to appeal the decision you have 30 days to send in your Petition for Review. The Washington unemployment office generally sends you all of the paperwork necessary for an appeal, however you may want to speak with an attorney to ensure you are filling out correctly. If you do not receive the paperwork or are unsure then you should definitely seek out the help of an attorney. If the decision is now in your favor then there is no need to appeal.