Shabbos, Chapter 17, Mishnah 4. This mishnah introduces us to the concept of muktzeh machmas chisaron kis: there are objects that would ordinarily be permitted to be moved as a keli shemelachto le'issur (a utensil whose primary use is forbidden), but because they are delicate and highly valuable, a person does not use them for any other purpose. No one picks up a valuable object to prop open a door, and that is the basis of this law.
"Rabbi Yose omer: kol hakeilim nitalin chutz min hamasar hagadol vi'yased shel macharishah":
"Kol hakeilim nitalin" - even utensils whose primary use is forbidden may be moved. Two utensils, however, are excluded from this rule:
"Hamasar hagadol" - a large saw used for cutting boards, which had to be kept in excellent condition, and people were not allowed to use it for any other purpose.
"Yased shel macharishah" - the part of the plow that cuts the furrow.
These two utensils are expensive, and therefore they are muktzeh machmas chisaron kis: they may not be used, and they may not be moved even "for a need or for their place," because a person removes them entirely from his mind, from any thought or expectation of using them on Shabbos.
"Kol hakeilim nitalin l'tzorech v'shelo l'tzorech":
From here the mishnah turns to discuss utensils whose primary use is permitted. Such utensils may be moved in two ways:
"L'tzorech" - when a person needs the object itself, such as a plate he needs to eat from.
"V'shelo l'tzorech" - not for the person's own use but for the sake of the object itself: if the utensil is lying outside in the sun and there is a concern that it will be damaged, a person may bring it inside, even though he is not using it for his own needs on Shabbos but only to protect the object. Since this utensil's primary use is permitted, it may be moved even for its own sake.
The view of Rabbi Nechemyah:
Rabbi Nechemyah disagrees and says: "Ein nitalin ela l'tzorech" - even a utensil whose primary use is permitted may be moved only for its regular use, and not for any other purpose. One who picks up a knife in order to cut bread with it is using it in its regular way; but one who picks up a knife in order to steady a wobbly plate or to prop something up is not permitted to do so according to Rabbi Nechemyah. The Gemara notes, however, that even according to Rabbi Nechemyah one may move an object for the sake of its place, when a person needs the space the object is occupying.
In summary: In this mishnah we learned the law of muktzeh machmas chisaron kis - the large saw and the plow blade, which because of their value a person removes from his mind entirely, and which may not be moved at all; and in contrast, utensils whose primary use is permitted, which according to the first tanna may be moved both for a need and not for a need, while according to Rabbi Nechemyah they may be moved only for their regular use, though even according to his view they may be moved for the sake of their place.