ETL stands for
extract transform load. It may refer to the process of migrating data from one source to another, or it may refer to a piece of
software that does the job.
Datawarehousing specialists use ETL tools to move a subset of data from a larger data silo to a smaller one (say, a data mart). Extract means that only certain records might be selected. Transform means that the data might be altered and tidied up along the way (as complicated as normalising a flat file into a relational format, or as simple as changing dates from Julian to Gregorian). Load means that the data is created in a new source, along with any necessary dependencies, primary keys, indexes or whatever a target platform requires of its tables.
ETL is a good example of an acronym for a process so simple that you didn't even think it warrented an acronym, so without even knowing what ETL means you think you are incapable of taking up work that demands ETL experience. Theoretically you can claim burning CDs as managing an ETL operation.