Every Friday (until Tuesday) you can come across a unique NPC called Xur, known from the first part of the game. The exact location of his appearance is random, but it is visible on the map under a dark green icon with the large roman number "IX". You can purchase legendary and exotic items of high level from him, which allows some players to easily reach higher power levels. Xur can sometimes also sell cosmetic items, such as various skins. During the few days of his stay his stock does not change, so you can expect interesting deals every week.
It is important that the basic currency at Xur's shop are Legendary Shards, which you will acquire by dismantling legendary items. It is worth preparing for the next arrivals of the trader by amassing decent amounts of them. Xur often has items that oscillate at a maximum power level of 270, which is why this is often a good way to improve your Power, if you have not exceeded this threshold yet.