Many airports have SID's (standard instrument departure) or DP's (departure procedure). Each SID is has several different transitions to get aircraft on their way. Their purpose is to reduce workload by having the aircraft fly a charted route to keep them away from arrival corridors, military airspace, housing areas, etc. w/o the controler providing vectors to every single one. Each SID/Transition also has altitude and often speed restrictions. If the controller had the plane do something other then what's written in the procedure, e.g. maintain a different altitude temporarily, he can tell them to resume the published procedure by saying "resume ____ transition" or "resume ______ depature ____ transition".
The SID's can often be found at
www.airnav.com. Just search for the airport in question.