What is the genetive case?

The genetive is the case of possession so it is used instead of the word 'von' with objects that are being possessed by a subject. (such as "My mum's flowers": flowers are the object being possessed my the mum -who is the subject of this phrase)

For example: 

The lady's bag: instead of die Tasche von der Frau 

die Tashe der Frau ( as die becomes der in the genetive and translates to of the) 

My brothers bag: instead of die Tasche von meinem Bruder

die Tasche meines Bruders (der becomes des, einer becomes eines in genetive)

So it is important to learn by heart the endings for the different cases! This comes with practice! :)