There are a good many sites that achieve page one ranking on Google that use no keyword or description tags at all, so take it with a grain of salt when someone tells you they are really essential for Search Engine Optimization. Again, each search engine has its own method for indexing -- some assign them considerable importance, thus it is best not to omit them.
Meta tags are lines of code that are hidden in web pages. The code information is not revealed in the web browser (but refer to the discussion of the description tag, below) but they are utilized by search engines to help categorize your web content. It is possible that you might choose to omit descriptions, or keywords, but your site won't look right if you don't put a "title" tag, since the web browsers will show it as "Untitled Page".
In order to grab traffic, years ago, meta tags would be "stuffed" with repetitive information, or information that has nothing to do with the web page. Search engines still check to be certain that the keywords are relevant;
however they don't give as much "weight" or importance to them, thereby penalizing this abuse. In any case, if you don't include meta tags, or they are filled with useless keywords, you will not rank well overall.