What is an extract is Tableau?

An extract is a compressed snapshot of your data held in Tableau's own columnar format rather than queried live from the source.

It is usually much faster, because the format is built for analytic queries — reading only the columns a query touches, compressing heavily, and keeping hot data in memory. It also works offline and removes load from the production system.

You are not obliged to take everything. The extract dialog lets you filter rows, hide unused fields, aggregate to a coarser level, or take a sample, all of which make the extract smaller and faster.

The trade-off is freshness. An extract is only as current as its last refresh, so it needs a schedule on Server, and incremental refresh where the volume warrants it.