In French, verbs can change their form to match the gender and number of the nouns or pronouns they describe. This is called 'agreement.' When we talk about past actions, we often use something called the 'past participle.' The past participle can change its form depending on how it is used in a sentence.
Let's break it down a bit more. When you use 'être' as the helping verb, like in 'She has gone,' the past participle changes to match the subject. So, 'allé' becomes 'allée' because 'she' is feminine. If you use 'avoir' as the helping verb, like in 'They have eaten the apples,' you only change the past participle if the thing being eaten (the apples) comes before the verb. So, 'mangé' becomes 'mangées' because 'apples' is feminine and plural.