On Error decides what happens when a runtime error occurs: GoTo a handler label, Resume Next to ignore and continue, or GoTo 0 to restore normal behavior.

Code

On Error GoTo Handler
...
Exit Sub
Handler:
  MsgBox Err.Description

• Handler

  On Error GoTo Handler ... Exit Sub Handler: MsgBox Err.Description → Errors jump to the handler; Exit Sub keeps normal runs out of it

  The standard pattern.

• Test existence

  On Error Resume Next Set ws = Worksheets("Maybe") On Error GoTo 0 If ws Is Nothing Then MsgBox "missing" → Tests for a sheet without crashing, then restores

  Narrow use of Resume Next.

• Cleanup

  Handler: Application.ScreenUpdating = True MsgBox Err.Number & ": " & Err.Description → Cleanup plus a message

  Restore settings in the handler.

• On Error Resume Next hides every error that follows; reset with On Error GoTo 0 as soon as possible.
• On Error Resume Next hides every later error; turn it off with On Error GoTo 0 as soon as possible.
• Resume Next in a handler continues after the failing line; Resume retries it.
• Err.Clear resets after handling inside Resume Next blocks.

• A handler without Exit Sub above it runs on every normal execution.
• Errors inside the handler itself are unhandled.