The doctrine of Judicial Restraint suggests that judges, while in the course of making legal decisions and setting precedents, should limit overturning laws unless that legislation is very obviously unconstitutional. It also has come to be synonymous with judges who adhere strictly to legal interpretations and existing decisions and laws, and who remain dispassionate and impartial when hearing cases.
Judicial Activism, on the other hand, is the exact opposite. Legally it means that a judge takes political and personal considerations into account when making a ruling or writing an opinion. In political exchanges, it has most commonly been associated with a ruling that is merely controversial or unpopular, especially by those on the conservative side of the political spectrum, and it has become something of a misnomer. It is sometimes, therefore, confused with judicial discretion - where a judge can take into account all of the circumstances of the case and sentence or rule with that in mind, as opposed to "legislating from the bench", which is much more rare in my opinion.