Shabbos, Chapter 7, Mishnah 3. The first mishnah in this chapter opened with a "klal gadol" - the major principle in the laws of Shabbos - and listed various rules concerning unintentional transgressions, a person who violates Shabbos inadvertently. Our mishnah also presents a principle, and although it is not a "great principle" like the previous one, it can be applied to the melachah of hotza'ah, taking things out and carrying them on Shabbos.
"Ve'od klal acher amru" - and they stated yet another principle:
The Sages stated an additional principle regarding the size of the items being carried out: what is considered significant and substantial enough to render a person liable on Shabbos? The item must be of a quantity that has importance to a person. A single grain of dirt clearly has no significance that would make carrying it out count as a melachah.
The two conditions for liability to bring a chatas:
"Kol hakasher lehatzni'a" - anything that is fit to be stored away, kept and set aside for future use. This does not refer only to the particular item, but to the fact that this is the type of item that people generally store away. It is not a useless clod of dirt, although certain types of dirt do serve a purpose and have a use.
"Umatznin kamohu" - that the quantity taken out is the quantity that people generally store away, meaning the significant quantity of that item.
"Vehotzi'o beShabbos - chayav alav chatas" - one who carries this type of item, in this quantity, on Shabbos is liable to bring a chatas offering for having desecrated Shabbos inadvertently.
"Vechol she'eino kasher lehatzni'a" - and anything that is not fit to be stored away:
In contrast, an item that is not fit to be stored away for later use or for a particular purpose, "ve'ein matznin kamohu" - or alternatively, even if the item itself is important but the quantity is insignificant and people do not generally keep such a quantity of it - "vehotzi'o beShabbos, eino chayav ela hamatzni'o" - and he carried it out on Shabbos, only the one who stored it away is liable.
In other words, the only person who becomes liable for carrying out such an item, or such a quantity, is the one who stored it away for his own personal use. Even though the general public does not use this item and has no interest in it, since he himself has an interest in this item or this quantity and designated it as something significant to him, he can become liable for it.
In summary: our mishnah adds a principle to the laws of hotza'ah: the measure that creates liability is determined by the importance of the item. Something that people generally store away, in the quantity they generally store, renders one who carries it out on Shabbos liable to bring a chatas. Something that people do not generally store away, or a quantity that is not generally stored, carries no liability when taken out - except for the person who stored it away for himself and thereby revealed that the item is important in his eyes.