Meta elements provide information about a given webpage, most often to help search engines categorize them correctly. They are used on a per page basis to help match user or searcher intent. Meta values are helpful for your Search Engine Optimization (SEO) because they decrease bounce issues. If you insert a Meta value on a particular page or blog post and a user searches for similar content, the user will see the "preview" of the content on Google and be able to make a better informed decision as to whether or not this link is going to take them to content they want to see.
Standard search engine etiquette is to not have more than 255 characters in either the meta keywords or description fields (including spaces and commas). You can input your values by separating each entry with a comma. Meta values entered here into this "Settings" area will be used for every page on your site UNLESS you edit a particular page and enter meta values at the page level. If you enter meta values at the page level, those will override these site wide values (not be merged with these).