Keywords in title tag for SEO
Title of the webpages that ranks in search engines are more frequently found to have keywords in them. The presence of keyword in the title shows the content related to it bears more relevancy to the search terms in the search engines. So, most of the SEO masters intelligently uses keywords that are most suitable for the title of the content.
Keyword stuffing consequences in title tags.
But taking advantage of this relation between title, keywords and search engine ranking many Black Hat SEO masters starts keyword stuffing in the title tags. They even starts to write titles with keywords which have no relation with the content, the whole trick has been applied to catch traffic to their sites. But soon search engines become smarter than Black Hat SEO masters and starts penalizing keyword stuffing in title tags.
Keyword frequency or repetition of keywords in title tag.
Most of the search engines including Google uses title as one of the criteria to set up the relevancy of the content with the search terms. Presence of keyword in title tag helps most in setting up this relationship. But it is a bad idea to repeat keywords in title tag. It's nice to have only single keyword within the title tag. It's even better to use the keyword towards the beginning of the title. Sometimes it is required to use the keyword twice to complete a phrase in the keyword but more than two keywords seems keyword spamming.
Three or more keywords in the title tags not only reads spam to the readers but it gets easily traced by smart search engines and will be penalized by search engines for keyword stuffing.