Two views of what the fruit that the hearer of the word bears refers to:
The fruit refers to sanctification (Luke 8:15; Galatians 5:22), a foretaste of the great harvest: the perfection that will begin when Jesus returns (Matthew 13:8; see also Romans 6:22).
What the fruit refers to, and what the difference in yield signifies, Jesus does not say; we only hear of the difference—but fruit is there. This verse implies that while the quality of the soil determines whether or not there will be fruit, the seed is still the source of the fruit.
Interpretation 1:
The fruit mentioned in the parable of the sower refers to sanctification (Luke 8:15; Galatians 5:22), and this sanctification is a foretaste of the great harvest—that is, the perfection that will begin when Jesus comes in glory (Matthew 13:8; see also Romans 6:22)1
Interpretation 2:
What the fruit referenced in the parable of the sower refers to, and what the difference in yield signifies, Jesus does not say; we only hear that there is always a difference—but fruit is there. In this verse there is a shift of subject, which shows that although the quality of the soil determines whether or not there will be fruit, the fruit nevertheless springs up from the seed.2
23 As for what was sown on good soil, this is the one who hears the word and understands it. He indeed bears fruit and yields, in one case a hundredfold, in another sixty, and in another thirty.”