Conversion rate is a term that represent how many of the visitors to your website take action on the content you provided to them.
This can include any product, or offer you have on your website.
In simple terms, conversion rate can be considered as a turn-out, which is nothing but a consideration of the amount that took your offer when you made available to them.
For instance, if you provide an offer on your website for a discount coupon in exchange of their email addresses, then the conversion rate is the percentage of the people took that offer from the total number of the people that visited/seen that offer.
Tracking program such as Google Analytics can be used to know about the conversion rate of your deal or offer that you want to know.
Apart from that you can even calculate it manually by just knowing the number of people visiting your website and the number of people accepting that offer, just divide the later by former and multiply it by 100 and that's how you calculate the conversion rate.
One can increase conversion rate for their site, by working on the best marketing methods like presenting your content in best possible way, reaching the right targeted audience etc.