The behavior blocker can be configured in the resident protection settings / Standard Shield / Blocker). If you have "Opening file for writing" enabled, it looks for files (the type is again configurable, I don't know your settings) being opened for write access. Unfortunatelly, some programs open the files for read+write access even when they don't intend to write there (yesterday, there was a similar topic; if you try to access "Properties" of a file, some microsoft library will do exactly this thing).
So, it is possible that you get such a warning, for example when you start some programs. You are not saying what operating system you have... it's a little strange that avast detected such an operation in itself (it shouldn't) - but it's possible that some third party library gets loaded, causing such behavior...
What OS do you have? What is the name of the DLL that's announced?
If you were getting these warnings out of nothing (if the computer was idle), then it would be really suspicious. When you're getting them when starting some program... well, hard to say, but it may not mean anything serious.
As for the buttons on the popup window - Yes means "allow the operation", No means "block the operation", and Ignore means "allow the operation and similar operations of the program in the same session".
I personally would rather recommend you to disable the "Open for writing" behavior blocker setting and rely on the rest of standard shield (maybe set executable extensions to be scanned on open). That would get rid of the warnings.