Please explain this paragraph from Sec 49 of the Revised Corporation Code of the Philippines or RA 11232.

Notice of any meeting may be waived, expressly or impliedly, by any stockholder or member: Provided, That general waivers of notice in the articles of incorporation or the bylaws shall not be allowed: Provided, further, That attendance at a meeting shall constitute a waiver of notice of such meeting, except when the person attends a meeting for the express purpose of objecting to the transaction of any business because the meeting is not lawfully called or convened.