on the FRED website where do i find definitions of the variables they give charts of? eg M3?
how can we see precisely how it is defined, measured?

https://fred.stlouisfed.org/series/MABMM301USM189S?fbclid=IwAR1lwaCbKUX16L1meOk6iZvloYOW0F7pIjmq9I4sb5VfvhhLhj8lsVTlpxg
The bottom of these indicator/series pages on FRED always lists the source. In this case, it's the OECD. If you dig around a bit on their site, and find where they host the data, there is always some meta-data attached to these that explains pretty much everything you would be looking for normally. Otherwise, they also list an email and you can ask them directly--which I've done on occasion.  They're slow to respond, but get back.  

For your specific question, for example, if you go to https://stats.oecd.org/, click on the 'Finance', 'Monthly Financial Statistics', 'Monthly Monetary and Financial Statistics' and the select 'Monetary Aggregates - Broad Money M3', it pulls up all the M3 data. On the right-hand side, there is a window you can expand that has all the metadata that answers your question.