Datadog is a software as a service (SaaS) data analytics platform that provides monitoring services for cloud-scale applications. It collects data from servers, containers, databases, and third-party services to make your stack observable, and has a flexible monitoring/graphing interface that allows you to display complex graphs without coding.
Datadog is considered to be a best-of-breed monitoring solution and has a lot of integrations out of the box, but also features a heftier price tag compared to other tools in the category.