URL variables capture dynamic parts of URLs and pass them as function arguments:

@app.route('/user/<username>')
def show_user(username):
    return f'User: {username}'

@app.route('/post/<int:post_id>')
def show_post(post_id):
    return f'Post ID: {post_id}'

@app.route('/path/<path:subpath>')
def show_subpath(subpath):
    return f'Subpath: {subpath}'

Variable types include string (default), int, float, path, and uuid.