First of all, what is social signals?
Social signals are shares of your link(s) on social media websites like Facebook, Twitter, Google plus, Pinterest etc.... All further shares, likes, pins, repins, retweets etc. are counted as social signals too. Most of this links are no-follow, but don't be discouraged because they are very important too. Not just for your audience, like followers or social friends, but for Google crawlers and algorithm too.
So how does it help your SEO in order to boost SERP?
Even if this links on social media sites are no-follow, you need to know that most social media sites are high authority sites and they are being crawled by search engines frequently. So, perhaps Google crawlers will notice your link shared on some or many social sites, they will also notice all likes, retweets, repins, shares... That's how their algorithm will notice how real people engage with your post and simply said, the more people engage with your social posts, Google will rank your post higher in their SERP because algorithms recognize it as something what people really like and means useful and worthy to people.
Simply share your posts on major social media sites and you will notice boost of your rank. There is more tactics to add to this, but this is simple explanation how and why to start getting social signals.