Meaning of "if you only knew" in english translation of 2:184

I am not a muslim, so I apologize for any mistake I might make while writing this question. I have recently started reading an english translation of the Quran, and there is a recurring phrase I am not sure how to interpret: "if you only knew". It occurs in a few places in Al-Baqarah, for example here, in 2:184: > [Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] – then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] – a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess – it is better for him. But to fast is best for you, **if you only knew.** (The translation here is Sahih International.) I need help understanding what is meant by this phrase. What would change, if we knew what?