A distributed denial of service (DDoS) attack is when an attacker floods a web server or an Internet connection with more requests than it can handle. This flooding of requests causes the service to become unavailable, and the company or person experiences an outage. Cloudflare now allows paid customers to create notifications that warn them when their sites are under a DDoS attack. There are different DDoS types that you can receive – HTTPS DDoS and L3/L4 attacks. The availability of each type of alert depends on what service you are using.
Source: https://www.bleepingcomputer.com/news/technology/cloudflare-can-now-send-ddos-alerts-for-sites-are-under-attack/