Peek() and Previous() have a basic difference when you use a FILTER (where clause) in load. Peek() operates on OUTPUT of source and Previous() operates on INPUT of source. For better understanding please refer this
Previous refers to the data coming from your DB, data source. Peek refers to the values you already have stored in QlikView data model, with the transformations that you have made.
Peek can be used from any table, any record being -1 the last one, (default), 0 the first record of the table, 1 the second, -2 the before last, -3 the before, before last, etc.