Definition statement

[Patio87]

Im am learning python right know and I have come to a chapter called functions
and procedures. It goes into detail about the "def" function. I canoont seem to
understand what it does. can someone please tell me what it means. If it is any
hel the book is "The Quick Python BOOK" by Daryl harms and Kenneth McDonald.
For some reason on the cover there is a Women on it. Please help me