Lookup and use case



I am new on using Anaplan. I was looking for use of lookup between source, mapping table and target. I read below statement but could not understand properly.

"If your mapping table contains the Target List and maps to a Source list-formatted line item or list-formatted property, then use LOOKUP in your formula." 

Does it means source and mapping table must have same list as dimension and dimension used in mapping table must be used as line item in target module. 

Any explanation would be helpful. 

Thank You