Not quite, it puts in place a timeline to fire the start gun, which is a big deal.
Actually, one alternative was to declare that the United Kingdom is no longer part of the EU. Afterwards, all relations between the U.K. and the EU would have to be negotiated de novo. The U.K. would not be obligated to pay any separation fee, accept immigrants, coordinate tariffs, tax law, or regulations with the EU.
If the U.K. took that route, the EU would have no legal standing of any sort on U.K, soil, laws. Declaring Article 50 is considered more polite, like filing divorce papers vs. tossing shoes into the street. Either is legal under U.K., EU, and international law.