why call dirty pages on checkpoint

Regarding  "checkpoint
flushes changed pages (“dirty pages”) from cache to the data portion of the
database
on disk " I don't know why call the changed pages "dirty pages"  ?

Regarding "SQL Server will flush only changed pages for which the change
was already recorded in the transaction log"   the "flush"  in the sentence what's that mean?  what's the detail on flushing?

thanks advanced !

if someone can explain ,i will be very appreciated

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