There is a (limited) list of verbs that conjugate with être in compound tenses - most only with avoir, some only with être, and a few can be with être or avoir, depending on usage/meaning. The lesson you have linked is quite advanced - the question you are asking is from earlier sections of development  - it may be better to work through the levels and get the basics before jumping to more advanced lessons.  

Conjugate coming and going verbs (+ être) in the compound past in French (Le Passé Composé)