The update frequency of your website does not directly effect your search engine rankings, but it may prompt search engines to crawl your site more often, and it may keep your readers engaged with your website.
If you are just looking to increase your rankings with the search engines, than it will be far better to work on the quality of your content, rather than the frequency that you post. You will want to post something that leads to more visitors automatically, and have the ability to catch the attention of your existing readers at the same time.
Post frequency has little to do with your rankings, but while post frequency doesn't have any direct ranking effect, it may still work to get new content indexed more quickly.
Old mistake, same results!
Its not really relevant how this idea came to be considered and practiced by so many SEOs, but you could easily speculate that someone noticed their website getting crawled by Google more often, and therefor concluded that the update frequency had an impact on their search engine rankings. Another possible explanation, would be that, by posting a large number of updates, in a short period of time, these SEO wannabes managed to improve their domain authority and keyword profile so quickly that it resulted in a large increase in visitors to their website – again, this could indicate that the update frequency had something to do with it – but what really happens, is that their site just appears more juicy by having more content.
As you can see, working with SEO is not for everyone – there are still so called SEOs recommending you to use the meta keywords tag – likely not because they don't know that it doesn't work, but more likely in an attempt to stand out in the crowd.
What with this update frequency then?
As already mentioned, the update frequency does not effect your rankings directly. Google also covered this officially in one of their webmaster videos. Basically the update frequency can work to keep your visitors engaged with your site, maybe course them to come back – but it can also have the opposite effect, if all you post is junk to boost your update frequency.
The update frequency of your website may however effect your crawl rate, search engines might chose to crawl your site more often, if it is frequently updated. This means that your new content will be indexed faster than otherwise – but this will not effect your rankings directly – any efforts at improving your rankings should be directed towards: quality of content, backlings, and things like that.