In Data item you have to select the tag for desired calculation mode and in expression you provide Performance equation expression to check the output.
Calculated data in PI Datalink is used to calculate the values based on PI Point or PI AF attributes values based on our expression.
Data Item- This will be your PI Point or PI AF Attributes for which you want to calculate the values.
Expression- Its a Performance equation you want to calculate.
For more details please check the below link,
Just complementing Siddhartha, Expression is a way to get data from data item(s) using performance equations, in other words, you use Expression to incorporate mathematical operations and calculations over PI Tags and AF Attributes.
Example of an Expression: TagVal('VPSD.OSIsoftPlant.PL2.MXTK2.Flow Rate') - TagAvg('VPSD.OSIsoftPlant.PL2.MXTK2.Flow Rate', '*-24h','*')
You can find more information, example and exercises on Visualizing PI System Data course workbook (available to download for all customers on this link), section 8.6.
Hope this helps!