SET PAGE META VIA BLOCK
You need to place this code inside your _prepareLayout() function in your Block file. Below is the complete function.
public function _prepareLayout()
{
//set page Meta's
$this->pageConfig->setKeywords('Hello Metakeyword');
$this->pageConfig->setDescription('Hello Metadescription');
return parent::_prepareLayout();
}
SET PAGE META VIA LAYOUT XML
Open your xml file inside your layout folder and place this code above the body tag.
<head>
<meta name="description" content="XML Hello metadesc"/>
<meta name="keywords" content="XML Hello keywords"/>
</head>
NOTE: When you try to add title via both the methods first preference will be given to the block method(php file).