The "five words" is not a reference to something in particular and is actually meant to be arbitrary. The statement is meant to show how much more important it is to speak something understandable than to speak in tongues.
If we consider the verse in its context it will be easier to understand. In particular consider 1 Corinthians 14:15-19 (ESV):
15 What am I to do? I will pray with my spirit, but I will pray with my mind also; I will sing praise with my spirit, but I will sing with my mind also. 16 Otherwise, if you give thanks with your spirit, how can anyone in the position of an outsider say “Amen” to your thanksgiving when he does not know what you are saying? 17 For you may be giving thanks well enough, but the other person is not being built up. 18 I thank God that I speak in tongues more than all of you. 19 Nevertheless, in church I would rather speak five words with my mind in order to instruct others, than ten thousand words in a tongue.
Paul is saying that he would rather speak a little using his mind than speak a lot in tongues because speaking in tongues does not provide a benefit to others nearby. They do not know what you are saying, and therefore they cannot agree or be "built up" as Paul says in verse 17. Because Paul was an apostle sent specifically to the gentiles, he has his mind focused on how he can better disciple others around him. Speaking words that others can understand is far more effective in discipling than speaking in tongues.
Discipleship is after all the primary mission of the church according to Jesus as recorded in Matthew 28:16-20:
16 Now the eleven disciples went to Galilee, to the mountain to which Jesus had directed them. 17 And when they saw him they worshiped him, but some doubted. 18 And Jesus came and said to them, “All authority in heaven and on earth has been given to me. 19 Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.”