The code is using substr as an lvalue. That is, on the left-hand side of the assignent operator '='. What that does, is it replaces the substring denoted by the parameters to substr, by whatever is assigned.

So, in your example, substr( '20050608', 2, 5 ) represents the sub string '..05060.'.

The value being assign is '2005' . '05'

The result is that dots in '20.....8' get replaced with '200506' giving '202005068'.

But then, maybe the question you're asking is "Why is the code doing this?" -- in which case: Sorry, I have no idea, it doesn't seem to make much sense.

Maybe it's a coding error and the intent would be clear from some more context?

Example: It might be an (erronously coded) attempt to convert US-style date formats (month/day/year) to Euro-style (day/month/year), but without context there is no way to tell.