Flask at first run: Do not use the development server in a production environment

I installed the Flask plugin in PyCharm Community Edition and I just have this simple code in my flask app:

from flask import Flask
app = Flask(__name__)

@app.route("/")
def index():
    return "<h1>Hello!</h1>"

if __name__ == "__main__":
    app.run(debug=True)

And I get this message:

WARNING: This is a development server. Do not use it in a production deployment.
Use a production WSGI server instead

* Restarting with stat
* Debugger is active!
* Debugger PIN: 123-456-789
* Running on http://127.0.0.1:5000/

Why am I getting this error when I run Flask?

_{A previous version of the message read "Do not use the development server in a production environment."}