I guess English learners use some phrases like "I don't know" and "I don't understand" frequently. To emphasize that, people would use some adverbs, e.g. really.
Adverb usually comes after the auxiliary verb and before the main verb, so, that expression becomes
I don't really know ...
However, another option seems to be used as common
Do they mean the same thing? Are they interchangeable in any cases? Could someone please give a hint? Thanks in advance.