From the same page where the question was copied/pasted:
What is a concurrent connection?
A connection is a measure of the number of users that are using your
app or site simultaneously. It's any open network connection to our
servers. This isn't the same as the total number of visitors to your
site or the total number of users of your app. In our experience, 1
concurrent corresponds to roughly 1,400 monthly visits.
Our Hacker Plan has a hard limit on the number of connections. All of
the paid Firebases, however, are “burstable”, which means there are no
hard caps on usage. REST API requests don't count towards your
connection limits.
Data transfer refers to the amount of bytes sent back and forth between the client and server. This includes all data sent via listeners--e.g. on('child_added'...)--and read/write ops. This does not include hosted assets like CSS, HTML, and JavaScript files uploaded with firebase deploy
Data storage refers to the amount of persistent data that can live in the database. This also does not include hosted assets like CSS, HTML, and JavaScript files uploaded with firebase deploy