Some verbs have a different meaning when they are used in the preterite and the imperfect tense. Saber is one of them.
In this sentence from the "Julieta" reading exercise, this sentence: "Siempre supe que había algo importante en tu vida..." means "I always knew for sure/was certain/ was very aware that there was something important in your life..." Supe has an extra nuance to the more general meaning "saber" (to know).
Having said that, it is actually the word "siempre" in front of supe that brings that specific meaning.
As a bit of extra information, saber in the preterite also means "found out", for example:
"Aquel día supimos toda la verdad" (That day we found out the whole truth)
Verb Querer (to want) is another one affected by this change in meaning. Here is a lesson about this in case you haven't come across it.
Un saludo cordial