Linked sheets are clones of other sheets in the timeline, they will inherit all pixel modifications made to the original sheets and vice-versa.
You can use this icon the same way as the other duplicate icons and drop the duplicated sheets anywhere in the timeline.
When you are on a linked sheet, the information “Linked sheet” is displayed in the bottom bar and other linked sheets are highlighted in purple, telling you that any change will affect the other linked sheet(s).
The linked sheets can display corresponding numbers to help you visualise pairs in a quick glance. The numbers correspond to their order of creation.
You can remove the display of these numbers in the Timeline settings, switch off Show symbols for linked sheets.