"26. It is by no means self-evident that this character of direct and universal exchangeability is, so to speak, a polar one, and as intimately connected with its opposite pole, the absence of direct exchangeability, as the positive pole of the magnet is with its negative counterpart. It may therefore be imagined that all commodities can simultaneously have this character impressed upon them, just as it can be imagined that all Catholics can be popes together. It is, of course, highly desirable in the eyes of the petit bourgeois, for whom the production of commodities is the nec plus ultra of human freedom and individual independence, that the inconveniences resulting from this character of commodities not being directly exchangeable, should be removed."

This is saying that commodities have two things (poles):

1. The one having the "character of direct and universal exchangeability", which is value. Value is interchangeable across various commodities

2. The "opposite pole, the absence of direct exchangeability", which is use-value. Use-value is not interchangeable; a coat provides warmth; bread provides nourishment; those are not interchangeable.

All commodities have the "character" of value "impressed upon them". The pope metaphor obscures more than it illuminates and I would've edited it out had I been the editor.

The last sentence is saying that the petit bourgeois benefit from having an easy way to exchange things on the marketplace. If there were no universal measuring-stick (i.e. value) we would have to barter and a merchant/trading class couldn't emerge.

"Finally, the form C gives to the world of commodities a general social relative form of value, because, and in so far as, thereby all commodities, with the exception of one, are excluded from the equivalent form. A single commodity, the linen, appears therefore to have acquired the character of direct exchangeability with every other commodity because, and in so far as, this character is denied to every other commodity"

"a general social relative value of form" means value. It is "general" because it is common to all commodities. It is "social" because it is used to trade between people. It is "relative" because it is used to compare linen-to-coats not linen-to-linen.

The bit you quoted is related to this other bit: "Since no commodity can stand in the relation of equivalent to itself, and thus turn its own bodily shape into the expression of its own value, every commodity is compelled to choose some other commodity for its equivalent, and to accept the use value, that is to say, the bodily shape of that other commodity as the form of its own value." Marx is saying that "all commodities, with the exception of one, are excluded from the equivalent form" meaning all commodities except linen are non-equivalent to linen. The linen "acquired the character of direct exchangeability with every other commodity" by getting value on top of its use-value.