A typical ETL job pulls orders from an ecommerce platform, contacts from a CRM and transactions from accounting software, cleans and standardizes fields such as dates and currencies, then loads the result into a data warehouse where it can be reported on together.
The common variant is ELT, which loads raw data first and transforms it afterward inside the destination system, taking advantage of the processing power modern warehouses offer. Either way, the transform step is where most of the real work and the real bugs live, since it has to reconcile the different ways source systems represent the same fact.