Oversimplified definition of Aggregator design pattern is; pattern which helps in combining one or more messages to create a aggreagated message. e.g if you've a system where you get n orders from a vendor in a day ; at end of day you want to combine all these orders to create a single order; you need aggregator pattern
There are lot of examples how we can do this in BizTalk using Listen shape (with one branch having delay and other receiving messages)
and once the delay timesout a new message is created with all received once. We can find more information here
The other and simpler approach could be using Append option in Send Port configuration. You can find this in send port properties-> transport->AddressURI-> Copy Mode. FileName should be unique and not with variable like %MessageID%.txt etc
So once we have this setup biztalk will append all records to same file which is your aggregated message.
Whenever we want to use this file we can simply cut this file to new location and biztalk generates a new file.
Cheers
Vishy