The Arabic word Al-Fidya or Fidyah is to feed a poor person for each day if someone abstains from fasting or break one's fast in Ramadaan, due to legal excuse. One should feed (either lunch or supper) on the average scale of that which one feeds his own family.
One can either feed a poor person for each day individually or gather poor people according to the number of days that require al-Fidya and feed them all at once. [Fataawa al-Lajnah al-Daa'imah (10/198)]
Old men and women, who are incapable of observing fasts in Ramadaan, may recompense for it by offering al-Fidya. Abdullah Ibn Abbas (radhi allahu anahu) narrates, Old people are allowed to refrain from fasting. But for every missed fast, one should feed the poor and he is not obliged to make al-Qada. [Daraqutnee and al-Hakim] Al-Nawawi said: Its isnaad is hasan.
Shaykh Ibn Baaz was asked in Majmoo’ al-Fataawa (15/203) about an old woman who is unable to fast – what should she do?
He replied: She has to feed one poor person for each day, giving half a saa’ of the local staple food, whether it is dates, rice or something else. This is equivalent to approximately one and a half kilograms. This was the fatwa of a number of the companions of the Prophet (peace and blessings of Allaah be upon him), including Ibn ‘Abbaas (may Allaah be pleased with him). If she is poor and cannot feed others then she does not have to do anything. It is permissible to give this kafaarah (expiation) to one or to more than one person at the beginning of the month, or in the middle or at the end. 
Things that are forbidden when in ihram include removing hair from the head or body and clipping the nails. For each of these a FIDYAH must be offered. The individual has the choice between sacrificing a sheep; feeding six poor persons, giving each poor person half a saa‘; or fasting for three days. That is because Allah, may He be exalted, says (interpretation of the meaning): “and do not shave your heads until the Hady reaches the place of sacrifice. And whosoever of you is ill or has an ailment in his scalp (necessitating shaving), he must pay a Fidyah (ransom) of either observing Saum (fasts) (three days) or giving Sadaqah (charity - feeding six poor persons) or offering sacrifice (one sheep)” Quran Surah al-Baqarah 2:196
And because of the hadeeth of Ka‘b ibn ‘Ujrah (may Allah be pleased with him). When he needed to shave his head whilst in ihram, the Prophet (blessings and peace of Allah be upon him) said to him: “Shave your head, and fast for three days, or feed six poor persons, or offer a sacrifice.” Sahih al-Bukhaari, Vol.3: 1814; Sahih Muslim, 1201
It is not permissible to offer the fidya in the form of cash, and it is not valid to do so, because that is not mentioned in the texts. Rather it must be given in the form of food, as enjoined by the Prophet (blessings and peace of Allah be upon him).