Lineup importing allows you to take a previously created lineup that has been entered into a contest and instantly recreate it in another contest.
To do so, on the Lineup Builder screen, click on "Import Lineup".
The next screen will display all previous Tournament contests played along with their individual lineups for each. This list is limited to your last 10 contests and will include all entries from each.
From there you simply choose which previous lineup you want and click the "Import" button.
Do keep in mind that salary values and player statuses can always change so while you may have been within salary when submitting a lineup in the past you may have to make modifications when submitting the current one.
Once imported and any needed adjustments are made click "Submit" to enter that lineup. If you are over salary or do not have all position slots filled you will not be able to submit and the button will be greyed out.