At some point, you might want to remove some tasks from the project but you don't want to lose all data on them. So what you do is to archive those tasks. You can do that in a several ways.
Option 1 – By editing the task in list or kanban view. Click on the pencil to edit the task then click on the archive icon in the far right.
Option 2 – From inside a task. In the "more" (3 dots) menu.
Option 3 – From the multiple select menu. Shift + clicking a task will enter multiple select mode. And you will have an archive option on the top. This is also useful if you wish to archive multiple tasks at once.
Option 4 - Delete an entire list. When you delete a list all the tasks inside are going to be archived.
Where do all the archived tasks go?
Open the project settings menu by clicking on the "gear" icon next to project title.
Then choose Archived tasks. This action will open the archived tasks so that you can restore them if you want.
Note that if you delete a list you will need to select a new list for it the archived tasks before you can restore them.