The Moabites were generally enemies of Israel, and thus God excluded all Moabites, and their descendents for 10 generations, from entering "the assembly of the Lord". This is Deuteronomy 23:3:
The woman Ruth, however, was a Moabite who became the wife of the Israelite named Boaz. This is itself doesn't present a problem, since Ruth apparently never entered the assembly. The problem, though, is that Ruth and her husband Boaz were (via their son Obed) ancestors of King David, one chosen of God and a great king of Israel. This is Ruth 4:18-22:
This would make David a descendant of a Moabite, and thus he should have been excluded from the great role that he took on. He certainly entered "the assembly". One could say that God made an exception, though this is rather unsatisfying. The only other solution is to claim that the genealogy is not complete, but rather skips some generations, so that 10 generations actually passed between Ruth and David. It's clear that Boaz was Obed's actual father, and it's clear that Jesse was David's actual father, so this must occur between Obed and Jesse.
How plausible this is, is up to you. It's not the natural reading of the text, and I'm not aware of any other reason to believe that Obed isn't Jesse's actual father. Realms of Faith has a chronological list of Biblical events, that helpfully puts Obed and Jesse's births about 50 years apart, and thus rules out any such shenanigans. I can't vouch for its accuracy though.