Nouns in German have different genders. There are three words for "the": Der, Die and Das. The possessive pronouns have to match the gender of the noun. For example, der Apfel (the apple) would be mein Apfel (my apple). die Katze (the cat) would be meine Katze (my cat). The reason for so many words for your is because German has an informal (dein) and formal (Ihre) tense and also the plural (eure) form, like when you are addressing a group of people.

There are different forms of your (formal and informal).

If the word is feminine then a -e is added onto the end of the root (mein)
if it is masculine or neuter then it is just the root

'ihre' is her, not your: Das (that) ist ihre Mutter (her mother). It is 'ihre', not 'ihr' because 'die Mutter'. But: Das ist ihr (because der) Vater (her father). Das ist ihr (because das) Pony. I meet my neighbour's three children and ask them: "Wo ist eure (because die) Mutter?" - Where is your mother? I ask them: Wo ist euer (because der) Vater? Or: "Wo ist euer (because das) Pony?" If I meet one of these children then I ask: "Wo ist deine Mutter?" "Wo ist dein Vater?" But "Wo ist euer Pony?" because the pony doesn't belong only to this child but to the whole family or to his or her father and I don't exactly know to whom.

This is explained in the grammar "Tips and notes" in Duolingo's web version.
Advice: Read them always before you start a new skill.

• Open Duolingo's web version (www.duolingo.com) in the "Desktop mode" of the browser of your phone, tablet or Pc
  (maybe impossible in an iPhone with a small screen)
• many skills have grammar "Tips and notes" under their lessons
• just click on the skill "Possessive Pronouns" and scroll to the bottom of the Lessons page
  https://www.duolingo.com/skill/de/Possessive-Pronouns

A simple, more comprehensive explanation can be found in:
https://www.germanveryeasy.com/pronouns#Possessive-Pronouns