“Hold fast the form of sound words, which thou hast heard of me, in faith and love which is in Christ Jesus.”
2 Timothy 1:13

He says hold fast sound words you hear- it is akin to what the Bible says in Proverbs 4:20-21
“My son, attend to my words; incline thine ear unto my sayings. Let them not depart from thine eyes; keep them in the midst of thine heart.”
You don’t just hear the word, you make every conscious effort to put the word in front of you, to ensure the word is always in your mind; this means primarily that simply because you heard the word doesn’t mean you can’t forget the word and walk outside the reality of the word, considering that there is an adversary that is determined to ensure you do not walk in the reality of God’s word, he tells you hold fast what you hear; what is that word that blessed you? Write it down, meditate upon it, put it as a part of your every day cogitations; ‘keep them in the midst of your heart.’