From the Lotus Notes help:
@DbLookup( class : cache ; server : database ; view ; key ; fieldName ; keywords ) or
@DbLookup( class : cache ; server : database ; view ; key ; columnNumber ; keywords)
Which means you have:
"" : "NoCache" - This doesn't concern you right now
Second parameter is empty, so you are searching the current database
"IT-MTBP Info" is the name (or alias) of the view you are searching
You are searching for all values matching the contents of the field (or variable?) named Prt1_ProjNo
This call will return the contents of the 15th column from view "IT-MTBP Info" for all matching documents
Here is the Example page from Lotus Notes Designer help: LINK
Hope this helps. If you still have questions you have to ask more specifically.
edit: fixed editing