Niddah, Chapter 6, Mishnah 6. This portion of the chapter is built out of a series of general rules, each one stated in a pair: a category that always carries a second obligation along with it, and then the reverse direction, which turns out not to be true. Our Mishnah applies that pattern to two different areas of halachah: the obligation to separate ma'asros from food, and the ability of food to become tamei.
Here is the opening clause: "Kol shechayav bema'asros metamei tum'as ochlin." Whenever an item of produce falls under the tithing laws, so that a tenth has to be set aside before anyone may eat from it, that same item belongs to the class of foods that can receive tum'as ochlin, the form of tumah that takes hold specifically in foodstuffs. The logic is straightforward: if the Torah considers the produce significant enough to demand a tithe, it is certainly significant enough to count as food for the purposes of impurity.
Now the Mishnah turns the rule around: "Veyeish shemetamei tum'as ochlin ve'eino chayav bema'asros." The reverse does not hold. There are foods that are fully susceptible to tum'as ochlin and yet carry no obligation of ma'asros at all.
Which foods are these? Anything that does not grow out of the ground. The obligation of ma'aser applies to the produce of the earth, so foods that come from a different source are simply outside that mitzvah. Meat, cheese, fish and the like are all real food, and as food they can certainly become tamei with tum'as ochlin, but no tenth is ever separated from them.
So the two halves of the Mishnah fit together into one clean principle: susceptibility to tum'as ochlin is the broader category, and the obligation of ma'asros sits inside it. Everything tithed can become tamei as food; not everything that can become tamei as food is tithed.