This saying, "keep His word" means to do what it says. Listen to it, obey it, consume it, fill your mind and heart with it:
Psalm 1:2But his delight is in the law of the Lord,
And in His law he meditates day and night.
I shall delight in Your statutes;
I shall not forget Your word.
8 This book of the law shall not depart from your mouth, but you shall meditate on it day and night, so that you may be careful to do according to all that is written in it; for then you will make your way prosperous, and then you will have success.