Lock Dates:  ways to make the General Ledger READONLY for certain users, at specific times, matching when you finish working on your Accounting documents - Invoices, payments, Bills, etc.

Fiscal Years: ways to teach Odoo which dates you define as the end of your year, or the start and end of a custom fiscal year.

Fiscal Years are really only needed if you don't use the calendar year as your fiscal year.  For example, your "year" may end on May 31st.  So when you report on "2019" you mean April 1st 2019 to May 31st 2020. 

Fiscal Years/Dates are just for reporting (technically they also to tell Odoo when to clear out the P&L accounts (reset them to zero) for a new "year") but think of them as labels that just make it easy to run a report for "2019" instead of having to choose April 1st 2019 to May 31st 2020 every time.