The "hymnal skein" was, as the Seventh Doctor described it, "a force that holds all life together".
Ace compared the hymnal skein to "the Force" from Star Wars. While the Doctor noted that as an example of Hollywood taking from things they don't understand, he agreed that it was a useful analogy; he further described the hymnal skein as what Taoists call "the Way."
Li Chen Mei and Ginny were guardians of the hymnal skein. Mrs Harris could sense the skein but didn't have the understanding to manipulate it. (PROSE: Hymn of the City)