When should I use the meta tag keyword?

Fabrício,

The most important thing for your site is based on the definition of Title (mainly), Page Description and URL, and these elements should be unique for each page and should also include the keyword.

For example, if we're talking about ' lose but '. It would be a great practice for your URL to contain the word ' lose-weight -xxxxxx', as well as the meta description, eg 'How to lose weight in 2 weeks with formula xxxx 'in the same way as the page title.

You would not use meta tag on the page with several keywords related to your project, this can also be identified as 'Keyword Stuffing', plus all your keyword research and planning work is available to any webmaster or specialist of SEO.

If you can, watch this Matt Cutts video

I hope I have helped.