Who must give fidyah in Ramadan?

The Almighty God decrees:

“(The prescribed fast is for) a certain number of days. Whoever among you does not fast on those days because you are sick or on a journey, then (you should fast) the same number of days on other days. Those who are able to fast are also required to pay a fee (which is equivalent to the cost) of feeding a desperate person. ” (Al-Baqarah /The Cow 2:184)

The Arabic expression we have translated as “those who are able to fast” is clearly positive in meaning in Arabic, but scholars attempt to assign negative meanings to the expression just because the tradition says so.

Sticking to the correct positive meaning, the verse reads: “Those who are able to fast the Ramadan fast, are also required to  pay a fee (which is equivalent to the cost) of feeding a desperate person.

 In Arabic “fidya”, is a fee that should be paid in order to recompense a deficiency in the accomplishment of a worship[1]. Considering this meaning, it is completely logical that those who are able to fast must pay a fee to compensate for the probable deficiencies in their worship.
This fee is the sadaqat al-fitr that must be given before the Eid al-fitr.
For details of the issue, please see the following article:
[1]Raghib al-Esfahani, Mufradat, art. فدي]