Shepard's traumatized little brain...
The Cipher.
Getting in Shepard's head to explain stuff is hard.
The game is vague. The Wiki does not help. The BSN is... well, I'm not going there. xP
Shepard is the only human to have direct access to the Prothean culture. Shepard knew what sequence of keys to press to free Liara on Therum, and they knew what sequence of keys to press to activate the working Prothean beacon on Virmire and on the N7 mission in ME2.
Also, they knew what to do with the Prothean artifact on the Firewalker missions.
Did this knowledge come from gaining the Cipher? Or was it just dumb luck button mashing?
Disregarding the notion of 'because the Word of God said so' (because, really, where's the fun in that explanation?), I like the idea of the Cipher imbuing Shepard with certain amounts of knowledge from the Protheans (not all, just some). Liara is drawn to Shepard at first because of what Shepard knows about the Protheans. And how can you experience a culture if you don't know its language or the idea of its language?
But how much knowledge is too much? Where do you draw the invisible line that is disbelief? What rules are set at the beginning that say, "Shepard knows this, but doesn't know that. Shepard wouldn't know that because, they know this."
In other words, Fandom: Opinions?
From the Wiki: It is the cultural context, knowledge and experience of the Prothean race. This unique racial perspective allows a mind to comprehend information as a Prothean could, though it takes time for a non-Prothean mind to become accustomed to it. Without the Cipher, the images in the beacon's vision don't make sense to non-Protheans who lack the necessary attitudes, mindset, and cultural experience to correctly interpret them. Describing the Cipher is impossible; it would be like trying to explain colour to a creature without eyes.
(no Cipher tag... have we never had this discussion?? Woo!)