We are studying Maseches Nazir, Chapter 1, Mishnah 6, which deals with a person who set the length of his nezirus according to the length of a journey.
The Mishnah opens: "Hareini nazir mikan ad makom peloni" - a person accepts nezirus upon himself from the place where he is standing now until a certain place. The Gemara explains that this refers to someone who has already set out on the road, meaning he has embarked on a trip that will clearly take some time, and he wishes to be a nazir for the entire journey. Perhaps this stems from concern over the dangers of travel, and he therefore accepts nezirus upon himself.
Estimating the Days of Travel:
When he uses this expression, "Omdin kamah yamim mikan ad makom peloni" - we estimate how many days of travel are required from the place he set out until that certain place, and according to this estimate the length of his nezirus is determined:
"Im pachos mishloshim yom" - if the journey takes less than thirty days, he is a nazir for thirty days, which is the standard term of nezirus.
"Ve'im lav" - if the journey is not less than thirty days, but rather thirty days or more, he is a nazir for the number of days of the journey. The length of his nezirus is set according to the number of days it takes to reach that place: forty days, fifty days, or any other amount.
In summary: In this Mishnah we learned that one who accepts nezirus upon himself from here until a certain place, referring to someone who has already set out on the road, we estimate the days of travel: if it is less than thirty days he is a nazir for thirty days, and if it is thirty days or more he is a nazir for the number of days.