Wechat is like Facebook, WhatsApp, Insta etc. (Western social media is blocked in China, after all), a major domestic e-business platform, the "Chinese PayPal, GooglePay, ApplePay" etc. in China (it started as a regular messenger app and morphed into this Uber-app for payments and the other stuff with time). It's virtually impossible now to exist as a Chinese citizen w/out Wechat.
For us Westerners, it is interesting for two things: For the bachelors among us, it apparently is useful for meeting Chinese women (because it's also their "Tinder", so to speak, since it's Insta, FB, Twitter and all in one
) and the other thing is: to get access to Wechat's own e-commerce platform with dealers and merchants or any other vendors in a magnitude that apparently dwarfs Aliexpress or Dhgate (although I believe you need to be fluent in Mandarin to use its full potential, or at least be efficient in using a translator
).
The big problem is: You can’t just sign up.
You need an invite from an existing user. And apparently, AFAIK, every user only has a limited number of invites per year.