Thrash metal appeared in the 1980s with bands such as "The Big Four": Metallica, Megadeth, Slayer and Anthrax. It's a sub-genre of metal music, characterized primarily by its high-speed and relentless brutality. Though these bands have a huge following, thrash is by nature an acquired taste, not designed to appeal to everyone. However, if you learn to appreciate the elements that appeal to thrash fans, you can understand the difference between a good thrash band and a bad one.
Start with the definitive albums by the definitive bands to get a sense of what constitutes thrash metal and makes it unique from the other sub-genres. It's a large genre containing a lot of bands, so you can avoid getting overwhelmed by taking in "the abridged history" of thrash with just three or four albums. Slayer's "Reign In Blood," Metallica's "Kill 'em All" and Megadeth's "Peace Sells" are ideal starting points for understanding thrash.
Pay particular attention to the level of musicianship exhibited by the players. Though metal is often dismissed by critics as noise churned out by talentless ingrates, many of them are world-class musicians playing hugely intricate and melodically complex pieces. Metallica's Kirk Hammett and Anthrax's Scott Ian are prime examples of musicians deemed to have elevated the art form.
Listen to the obligatory "slow, anthemic track" that appears on most thrash albums. If you're struggling to get around the speed and aggression of thrash music, most albums feature at least one slower song that gradually builds to a crescendo. If you find the volume and anger a little overwhelming, starting with songs such as Metallica's "One" or "Unforgiven" may allow you to appreciate the underlying melodies and counterpoints before the song explodes into cacophony.
Try to understand that all music is really a distillation of a given emotion. Music is meant to express everything from hope and joy to rage and despair. Everyone can understand the idea of being furiously angry with the world around them, for whatever reason. Thrash is a sonic interpretation of that anger. Tapping the drums in a light shuffle, turning down the guitars or singing in a velvety falsetto wouldn't really be true to the emotion behind the music. If nothing else, it's easy to commend quality thrash metal on its honesty.