The official vocabulary list of Metabase is available here: https://github.com/metabase/metabase/wiki/Metabase-Glossary.
To get a shorter list to get started, please find my definitions here below:
Collection: it is nothing more than a folder in which you will insert the different items that you will create.
Dashboard: a page on which you will insert your different data sets (named question in Metabase).
Database: it represents an external set of data that you can connect to Metabase in order to import data within it.
Format: also referred to type. It is the property of your data: date, text...
Metrics: they are types of data that you can make mathematics on.
Pulse: it is a synonym of a notification.
Question: in fact, they are simply queries to databases that you can save in order to save as widgets for your dashboard.
Segment: that's the possibility you have in order to filter your data.
SQL: Sequence Query Language, the most used database language in order to query databases. Without knowing it you will probably not go that far within Metabase.
Tables: the way data are displayed with columns and rows.
Visualization: the way data are displayed.
X-ray: X-ray in Metabase jargon means that guess what would be interesting to study in terms of data for you and are automatically selecting sets of data from your collection.