Kiddushin, Chapter 1, Mishnah 10. The preceding mishnayos dealt with the obligation in mitzvos and with different people in different places, and our mishnah closes the chapter with a discussion of the value of mitzvos in general.
One Who Performs a Single Mitzvah:
The mishnah states that a person who performs one mitzvah - "meitivin lo, u'ma'arichin lo yamav, v'nochel es ha'aretz" - good is done for him, his days are lengthened, and he inherits the land. This refers to a person whose merits and liabilities are evenly balanced against one another, half transgressions and half mitzvos, and that single mitzvah tips the scale in his favor. Once his merits outweigh his liabilities, good is done for him, his days are lengthened, and he inherits the land.
The Gemara explains that this is not referring to the physical land alone, but to the land of life - the eternal land of the World to Come.
Conversely, one who does not perform that single mitzvah and leaves himself in a state where the majority of his deeds are transgressions - good is not done for him, his days are not lengthened, and he does not inherit the World to Come.
"The Threefold Cord":
The mishnah continues and states that one who has these three things - "lo bimheirah hu chotei" - he does not readily come to sin:
Mikra - engagement in the Written Torah.
Mishnah - engagement in the Oral Torah.
Derech eretz - proper conduct in all that concerns his dealings with other people.
These three guardians protect a person from sin, as in the verse in Koheles that the mishnah cites: "v'hachut hameshulash lo bimheirah yinatek" - a threefold cord is not readily snapped. The cord is stronger because of the three strands that hold it together.
And one who has none of these three things is not even considered someone who contributes anything to the repair of the world on any level whatsoever.
In summary: In this chapter we learned about the value of a single mitzvah that tips the scale in a person's favor and earns him good, length of days, and an inheritance in the land of life of the World to Come, and about the three strands - Mikra, Mishnah, and derech eretz - that protect a person from sin, and that in their absence a person is not counted among those who build up the world.