Hi, Matthew. "Lo" is a pronoun, similar to "la", "le", and its plural forms. Is for the 3rd person and is used to refering something or someone previously mentioned as: -¿Has leído el nuevo libro de Marcos? -Sí, lo he leído (el nuevo libro de marcos) ayer. Is for something called "Elipsis"

Probably what is throwing you off is the location. Spanish puts pronouns in a different spot than English does (sometimes, other times it is the same). For example: ¿Lo conoces? = Do you know him? In this case, Lo is the pronoun "him", but it is before the verb. (Lo can also mean "it" if the thing is masculine) "Lo que" is often used as a unit, and in my mind I usually think of it as "that which".

"todo lo que tengo" = "everything I have" but to unpick the grammar think of it as "all of what I have" with "todo" corresponding to "all of" and "lo que" corresponding to "what" I'm not sure about "de lo que sientes" (my Spanish is intermediate) but I think it means "based on what you feel" or "from what you feel", so again "lo que" would correspond to "what". Similarly "depende de lo que sientes" would be "it depends on what you feel" because "depende de" means "it depends on" and "lo que sientes" is "what you feel". This is not like using "what" as a question word, of course. "lo importante es..." means "the important thing is..." or "what's important is...", i.e. "lo"+adjective is like "what's"+adjective or "the"+adjective+"thing" "Sabes lo importante que es" means something like "You know how important it is". But check with a native speaker to be sure!

abstract ideas / relative clauses become subjects in Spanish by adding (lo). Lo importante es que aprendemos.