Romans 15:28-29 meaning

Paul writes when I have finished this (v 28), that is, when he has completed delivering the financial contribution from the churches in Macedonia (northern Greece, Philippi, and Thessalonica) and Achaia (southern Greece, including Corinth) to the poor saints in Jerusalem, I will go on by way of you [Rome] to Spain (v 28). 

By saying he will put my seal on this fruit of theirs (v 28), Paul means he desires to see this offering of money (fruit) delivered safely to the poor among the saints in Jerusalem.

When Paul refers to the financial offering as fruit, he refers to the good deeds of the Gentile believers in Greece and Macedonia. Fruit is evidence of the characteristic nature of a particular tree. Scripture uses fruit as a word picture telling us that deeds reflect the heart, or nature of a person. The Gentile believers who were willing to financially support Jewish believers showed that their heart was turned toward God, therefore that deed was good fruit. 

Paul continues: I know that when I come to you [believers in Rome], I will come in the fullness of the blessing of Christ (v 29). This could mean that he has confidence that he will be richly received by the Roman believers, that his visit will be a blessing to them, that he will come with the blessing of Christ because he has chosen to walk in obedience to God, or it could mean all three. Either way, Paul is confident that his visit with the believers in Rome will be an encouragement to him and them (Romans 1:12).

Paul did make it to Rome, but not according to this plan. He was later taken to Rome under house arrest and was eventually martyred there. Acts 27 and 28 explain Paul's journey to Rome. Paul knew he was going to his death but gladly did so because that is what God led him to do (Acts 20:23).