In Cache write-through, data is written to the main memory through the cache immediately. The benefit of this approach is that it simplifies the design of the computer system, with the main memory always having an up-to-date copy of line data. When a read is requested, the memory replies with the requested data.
In cache write-back, data from cache is written in a "latter time". With this approach, sometimes the up-to-date data is in a processor cache, and other times in the main memory. In cases where the data is in the processor cache, then the processor must stop the main memory from replying to a read request because the version in the main memory would be outdated.