OK, so referral visitors is a statistic you will see in Google Analytics. This means a visitor that has come to your site from another website.
An increase in this number could be a great thing because it could mean that a listing you have on another website for example CheckaTrade or GuidesforBrides is working, but if your referral visitors jumps up unexpectedly and when you click through to see where these visitors are coming from sites that you don’t recognise like xyz.com or seo-platform.com then this is referral SPAM.
These ‘organisations’ send internet bots to your website to increase traffic. You think ‘oh this site is sending me loads of traffic, I’ll check it out,’ then one of two things happen, either they will offer you paid advertising to get even more traffic, or they will just target you for more and more SPAM.
There are ways to occlude this data from your Google Analytics reports, because they do skew your statistics, but in general if you do not respond they will stop within a month or two.
One good thing about increased Referral Visitors is that they have generally found you in search engines, so it means your SEO efforts are working.