Well if you are talking about ashWebSv.exe, it is the avast Web Shield provider that scans Internet Port 80 HTTP traffic.
It uses a localhost proxy where files are scanned before they are saved to your browser cache (temp internet files, etc.) any infected files can then be intercepted and stopped from getting on to your system.
I suggest a browse of the avast Help file, Resident Protection, Web Shield, etc. see image extract of help file.