A normalized measure of the CPU, memory, and I/O cost of each operation in Azure Cosmos DB. Reading a 1 KB document by its id and partition key costs about 1 RU. Throughput is provisioned in RU/s, with autoscale, or in serverless mode.
In DP-900 it goes together with the partition key: a good key spreads data and RUs evenly; a poor one creates "hot" partitions that limit throughput.
Cosmos DB Request Units (RU) is a concept that can show up on the exam. Check the full guide to see it in context.