Idea
Scrum is a pre-defined framework that contains organizations and practices to help teams be more agile. The main ones are a fixed iteration time (usually 2 weeks), a lightweight set of roles (scrum master, product owner), and some masses (standup meetings, demo, sprint planning, retrospective).
Although Scrum is very popular and has become a synonym to agile, I have found that it is rarely well implemented. Teams have a tendency to bound themselves to cargo-cult by following scrum, and stop asking themselves why they are doing things the way they do. The artificial timing of sprints is often pushing teams to reduce the cadence of delivery[1].
It might be a good tool to lower the rework of process definition, but to use with care
Links
[1]: Scrum is using timeboxing
The timeboxing of Scrum which can also suffer from Parkinson's law
Scrum is artificially setting the batch size to whatever fits in the sprint size. - this is one of the major problems I have with it.
References
Gergely Orosz / How Big Tech Runs Tech Projects and the Curious Absence of Scrumref None of the companies which ship use Scrum, most have "no formal process"