flash() a message in the fishy() view before the return. The message can have any content you want.

from flask import Flask, redirect, url_for, render_template, flash

app = Flask(__name__)

app.secret_key = "34fgrtrnhdyt465&&&&%%%98jghtd"


@app.route('/')
def index():
    return render_template("index.html")

@app.route('/fishy')
def fishy():
    flash("Howdy Friend")
    return redirect(url_for('index'))