Data entry is the actual act of entering or recording data, usually into a spreadsheet or database or some kind.
This is one of the most important and most tedious tasks in politics. Data is critical for keeping track of and understanding people, trends, circumstances, finances, events, etc.
When a constituent calls, if the staff has kept good records, then the legislator can look at all of the information that the staff have entered into the database and see: where the constituent stands on issues, services the legislator has provided in the past, and the status of the particular case work that he/she is helping the constituent with at the time.
Fortunately, the interns had finished entering all of the data from the fundraiser so that the Chief of Staff could see how much money they had raised and know how far they were from achieving their fundraising goals.