What relationship is my mother's mother's sister's daughter's daughter to me?

Let's work through this question step by step. 

The initial question is 'What relationship is my mother's mother's sister's daughter's daughter to me?'

Your mother's mother is your grandmother. So you can rewrite the question as 'What relationship is my grandmother's sister's daughter's daughter to me?'

Your grandmother's sister is your grand-aunt. Now the question is 'What relationship is my grand-aunt's daughter's daughter to me?'

Your grand-aunt's daughter is your mother's first cousin, which is...

Let's work through this question step by step. 

The initial question is 'What relationship is my mother's mother's sister's daughter's daughter to me?'

Your mother's mother is your grandmother. So you can rewrite the question as 'What relationship is my grandmother's sister's daughter's daughter to me?'

Your grandmother's sister is your grand-aunt. Now the question is 'What relationship is my grand-aunt's daughter's daughter to me?'

Your grand-aunt's daughter is your mother's first cousin, which is your first cousin once removed. Now the question is 'What relationship is the daughter of my first cousin once removed to me?'

The daughter of a first cousin once removed is your second cousin. 

Cousin relationships can be tricky to define. Just remember that first cousins share grandparents, second cousins share great-grandparents, third cousins share great-great-grandparents and so on. 

The 'once removed', 'twice removed', etc. is the number of generations between the cousins. So your mother's first cousin is your first cousin once removed (one generation above you). Similarly your first cousin's child is a first cousin once removed (one generation below you).