Personalized marketing is an approach that uses data and custom insights to communicate more relevant offers and engage customers more effectively. It can be one-on-one but usually it is done for separate customer segments. Tailored emails, email automation, retargeting are just some forms of personalized marketing.
Personalized marketing is based on customer behavior - offers similar products to what people bought, reuses campaigns that converted, continues a line of communication that worked with this segment before. The power of personalization is in its relevance - people get what they like and you get revenue in return. Customer engagement and retention are way higher than what blanket, one-size-fits-all marketing can do.