What exactly is 'fast fashion'?
Big fashion brands become big for a simple reason - people want the latest styles, styles often started by celebrity actors, music artists, or models.
The main problem is that these big fashion brands tend to charge a lot for their clothes, with huge differences between what you'd pay for a jacket in, say, Primark, versus what you'd pay for one from Gucci.
Some companies saw that they could emulate the latest styles from big fashion brands, then sell them for a fraction of the price - and thus 'fast fashion' began.
Why is this an issue though? The problem is that big fashion brands rely on a quick turnover for their expensive items, with a comparatively smaller group of wealthy individuals buying expensive items regularly.
Because fast fashion is always seeking to emulate the big fashion brands, you get a similar situation of relying on a quick turnover, only the profit per item is much less. So fast fashion needs a much larger audience in order to generate revenue.
If a much larger audience is constantly buying copies of whatever the latest styles are, a lot of clothes/shoes/etc. need to be made. In times gone by, you'd traditionally have two clothes-buying seasons (Spring/Summer, Autumn/Winter). Now, we effectively have 52 microseasons, one for every week.
What's actually wrong with more clothes being made?
The fast fashion industry needs to produce huge numbers of clothes to remain profitable, and this means using a lot of resources. For example, one kilogram of raw cotton requires between 10,000-20,000 litres of water, which equates to around 3,000 litres of water for a single cotton shirt.
Put this into the perspective of a global industry, which sells around 2 billion t-shirts every single year, of which a good chunk are cotton. You can start to grasp the astonishing volumes of water needed to keep this industry going.