Can someone please explain to me what "חפצך אסור חפצי שמים מותר" means. Tractate shabbat 150a?
If the Torah had said ממצוא חפץ, it would mean that any דיבור חול would be asur. But now that the torah says ממצוא חפצך, we learn that your 'things', that is, which are not mitzvot are asur, but חפצי שמים, which is anything that is a mitzva, is mutar. By the way, the gemara in the first part of the page gave a bunch of examples: calculations for money for the poor, or even for an individual poor person, and so on.
Isaiah 58:13 reads:
אִם-תָּשִׁיב מִשַּׁבָּת רַגְלֶךָ, עֲשׂוֹת חֲפָצֶךָ בְּיוֹם קָדְשִׁי
[I, God, will be happy] if you pause your feet on the Sabbath, and don't deal with your [business] matters on my holy day.
This is why it's not in the spirit of the day to talk business on Shabbos. However the Gemara says the prohibition is on "chafatzecha" -- YOUR business matters. You are, however, allowed to discuss business matters pertaining to God; e.g. all sorts of logistical matters concerning mitzva activities such as finding a trade school for your son, a husband for your daughter, or communal medical needs.
(This is echoed in the Friday-night song mah yedidus, the verse chafatzecha bo assurim)