Full Idea
Earlier Frege was interested solely in the content of our statements, not in our grasp of that content. His notion of 'sense' from 1891 onwards, has to do with understanding; the sense of an expression is something we grasp.
Gist of Idea
Earlier Frege focuses on content itself; later he became interested in understanding content
Source
report of Gottlob Frege (On Sense and Reference [1892]) by Michael Dummett - Frege philosophy of mathematics Ch.2
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.13
A Reaction
The important point must be that the later theory depends on the earlier, so we can hardly give theories of understanding, if we don't have a view about what it is that is understood.