Fast Fashion is the second largest polluter in the world after the Oil industry. Fast Fashion clothing brands make cheap clothing FAST, and often market to young (new) consumers. It is difficult to stop the production of such corporations that release new collections every season. These posters were made to raise awareness to just some of the many issues that arise from Fast Fashion.
The best thing you can do to help, is to reduce demand. Try to purchase used/secondhand clothing, and upcycle/mend the clothing you own. As it is, only 10% of clothing that is donated to these second hand stores gets sold, and unfortunately the rest is destined to end up in a landfill.
I highly recommend watching The True Cost documentary on such issues for more information.