To illustrate, let's look at summaries of top popular projects (by number of commits, because that's what Apache shows on stats page):
> Camel is an Open Source integration framework that empowers you to quickly and easily integrate various systems consuming or producing data.
> Apache Flink® — Stateful Computations over Data Streams [here image of taking events and DB as inputs of process]
> Airflow is a platform created by the community to programmatically author, schedule and monitor workflows.
??? Why would i need them?
> Apache Spark™ is a unified analytics engine for large-scale data processing. > spark.read.json("logs.json").where("age > 21").select("name.first").show()
> Lucene Core is a Java library providing powerful indexing and search features, as well as spellchecking, hit highlighting and advanced analysis/tokenization capabilities.
These tell or show what they do. Good.