Generate acronym from phrase
You should name your function more descriptively, maybe acronym.
You should wrap your code under a if __name__ == "__main__": guard to allow importing parts of your script from other scripts.
You should separate the user input from the logic.
Use docstrings to describe what functions do.
Use str.join together with a generator expression to join multiple strings.
Use the new f-strings introduced in Python 3.6.


Taking this gives you:



def user_input():
    """Get the user input for a phrase"""
    return input("Enter the phrase: ")

def acronym(words):
    """Turn a list of words into an acronym"""
    return "".join(word[0] for word in words).upper()

def main():
    words = user_input().split()
    print(f"The acronym for your phrase is {acronym(words)}.")

if __name__ == "__main__":
    main()