1. explain something in a clear and detailed way
set out your plan/reasons
The phrasal verb ‘set out’ has a number of meanings in social and professional situations. Firstly 'to set out' can mean to begin something or to start working in on something in order to achieve a specific result. If you 'set out' objects it means to put things where they can easily be seen or used by others.' Set out' can also mean to explain something in a clear, sequential or detailed way. So you can 'set out steps' or 'sent out instructions' for example.
2. begin a journey somewhere or begin doing something with a specific intention
I achieved what I set out to do
You can use 'set out' to talk about starting or beginning a journey, especially a long one.