Our Mishnah continues with the mitzvah of bi'ur, and specifically with regard to ma'aser sheini - more precisely: how this mitzvah applies to ma'aser sheini nowadays, when the Beis HaMikdash is not standing.
The rule for ma'aser sheini nowadays:
The Mishnah discusses one "sheyesh lo peiros shel ma'aser sheini bazman hazeh" - who has produce of ma'aser sheini nowadays. The rule here is that the produce is not eaten in Yerushalayim, and this is derived from the verse that compares ma'aser sheini to a bechor: just as the bechor, which is normally brought as a korban in the Beis HaMikdash, is not relevant when there is no Beis HaMikdash, so too ma'aser sheini is not eaten at a time when there is no Beis HaMikdash.
When the time of bi'ur arrives:
So what is done with the produce when the time of bi'ur arrives? May one simply dispose of it as is? The Mishnah says that it must be redeemed onto money - not destroying the produce itself directly, but transferring its sanctity onto coins. The Yerushalmi derives this halachah from the verse, that this is how one is to deal with the produce nowadays.
The difficulty:
At first glance there seems to be no logic in redeeming onto money first, since either way the end result is bi'ur - either the money will have to undergo bi'ur, or the produce itself will have to undergo bi'ur. If so, there is no compelling reason to convert the produce into money, since both of them require bi'ur in any case.
Fulfilling the mitzvah in practice today:
This halachah is in fact still fulfilled today. There was even an occasion to fulfill it in practice, and it is quite likely that this was the only time in history that the mitzvah of bi'ur ma'aser - removing the ma'aser - was performed in the state of Kentucky. Here is what happened. A father-in-law, who should live and merit a complete recovery, lives in Eretz Yisrael, but he spent Pesach in America and joined the family for the final days of the festival. He and the grandfather were picked up from the airport.
He looked at his watch and realized that it was getting close to sunset in Yerushalayim, on the eve of the final days - which is apparently the time when bi'ur is actually performed. He had with him money onto which ma'aser sheini had been redeemed, so the car got as close as it possibly could to the Ohio River and the money was thrown into it. At least, that is what was thought to have happened: no spot close enough could be found, so the money was thrown as far as it could possibly go. That, in any event, is how the mitzvah would be fulfilled in our times.
For the benefit of listeners who do not live in Cincinnati and do not know that the Cincinnati airport is located in Kentucky: the family does not live in Kentucky, but the Cincinnati airport is there, and since they wanted to perform the bi'ur as early as possible upon leaving the airport, they did so on the Kentucky side of the Ohio River.
In summary: in this Mishnah we learned that ma'aser sheini nowadays is not eaten in Yerushalayim, based on the comparison to the bechor, and that when the time of bi'ur arrives the produce is redeemed onto money and the money undergoes bi'ur - even though at first glance this seems unnecessary, since both the produce and the money require bi'ur. We also saw how this mitzvah can actually be fulfilled even in our times and even outside Eretz Yisrael.