The chapter before us deals with tzaraas that appears on a garment, on an article of clothing. The Torah lists three materials that can become tamei with nega'im:
Wool - and specifically classic wool that comes from sheep, not wool that comes from other animals.
Linen.
Leather.
A mixture of sheep's wool and camel's wool:
The Mishnah deals with a garment made from sheep's wool and camel's wool that were spun together, so that every single thread is composed of both types of wool. The status of the garment is determined by the majority in the mixture:
A majority of camel's wool in each of the threads: The entire garment does not become tamei, even if a nega appeared on it, since the status of the garment follows its majority.
A majority of sheep's wool in each of the threads: The article of clothing becomes tamei if a nega appeared on it, since here too we follow the majority in the mixture.
Half and half: Here there is no majority of camel's wool to nullify the minority of sheep's wool and render it insignificant. It turns out that the half of the thread made up of sheep's wool is considered significant, and we cannot ignore it. Therefore such an article of clothing becomes tamei if a nega appeared on it, because part of it is sheep's wool that is not nullified by another majority - and like any article of clothing made of sheep's wool, it becomes tamei with tzaraas.
A mixture of linen and hemp:
The same guidelines and laws apply to a garment whose threads were spun from a mixture of linen and hemp. Hemp comes from a different plant that grows from the ground, and in that respect it closely resembles linen, but ultimately it is not actually linen - and only a genuine linen garment becomes tamei with nega'im. For this reason, when it is mixed with another species, the law is determined by the very same criteria just listed in the Mishnah.
In summary: In this chapter we learned that only three materials become tamei with nega'im - sheep's wool, linen and leather. In a garment spun from a mixture of a species that is subject to tumah together with a species that is not, we follow the majority in each thread: a majority of camel's wool - it does not become tamei; a majority of sheep's wool - it becomes tamei; and when it is half and half it becomes tamei, since there is no majority capable of nullifying the significant species. And those very same criteria apply to a mixture of linen and hemp.