H2O is an open source machine learning platform that provides interfaces for R, Python, and Scala. It allows for data exploration, model building, and feature generation on large datasets using high performance machine learning algorithms. The document discusses how to get started with H2O in R, including downloading and reading documentation, exploring data, building models, and exporting models for real-time scoring. It also provides overviews of the H2O architecture and how algorithms are run in a distributed manner across an H2O cluster from R.