The title meta tag is the most important of the meta tags. Not only does it carry a fair bit of SEO weight, but it is the text that appears as a link to your page in the search engine results.
The description tag is an important element of your page when it comes to search engine optimisation. Although not essential these days, it is advisable to put one in.
The keywords tag is ignored by the majority of search engines these days and has little relevance with those that do recognise it. Despite this it is still common practice to include it in your head tags and it certainly won't do any harm if constructed properly.