Refers to a practice, publicly (and by many judges) considered improper, of trying to ensure that a proceeding goes to a sympathetic judge, either politically, or because of perceived bias towards a particular type of party, social, political or otherwise (for example pro-patent holder) or because of that judges particular practices (or non-practices) when it comes to speed of trial, control of discovery or lack thereof, inclination to sanction frivolous or poorly prepared cases, etc. Thus patent trolls are perceived as favoring courts which will allow a case to proceed for a long period, expensively for the defendant, before addressing early identified merits issues, which suits a party seeking a nuisance value settlement. Similarly, some judges are perceived as conservative or even extremely right wing, depending on how they reached the bench (in the US what President or Governor appointed them.)
Judge shopping is easiest when a court has only one active judge, for example a one-judge division. Where the court uses a wheel or rotating assignment system, strategies are, allegedly, more cunning. Judges tend to deplore any suggestion that judge-shopping happens, but, realistically it does happen, because of perceived or assessed bias of certain judges.