I am going to merge this with a similar thread, you should see your questions about "repeater" and its properties answered above.
Your image includes Item and its properties as well. This means you are in a Repeater or an Action that acts on a Repeater. Item is the current Item you are working with. So if you do Set Text of Shape to [[Item.Name]] in OnItemLoad of the Repeater each Shape in turn will be set to its corresponding rows "Name" column.
The properties after Item are executed on an item.
index: Gives the corresponding index of the Item in the Repeater (similar to the row, but Sorts, Filters and Pagination matter)
is_: Returns "true" or "false" representing whether the Item is first, marked, visible, etc...
repeater: returns the parent Repeater of this item which can then in turn use the Repeater properties that follow it.
There are 2 ways to access Repeater properties.
From inside a Repeater do [[Item.Repeater.visibleItemCount]]
From outside a Repeater or referencing a different Repeater. In the expression (fx) editor, set a Local Variable (myRepeater) to a Repeater Widget. Then use [[myRepeater.visibleItemCount]].
Item vs TargetItem
This was posted elsewhere, but probably fits better here. The distinction between Item and TargetItem.
Item: Used to reference the current Item you are working with. Only available inside a Repeater. This is the most common.
Example: Set Text of Button Shape to [[item.Column1]] of the Data Set.
TargetItem: Used to reference an item that is Marked or in another Repeater.
Example (using Marked Rows and Interaction - Repeaters -> Datasets -> Update Items):
(on a click copy my data to another Marked Item) use Item
(on a click copy another Marked Item's data from one Column to another) - use TargetItem
You can also use TargetItem to reference Items in another Repeater or from outside a Repeater.