Reading fluency is defined as the ability to read with speed, accuracy, and proper expression. In order to understand what you read, you must be able to read fluently whether you are reading aloud or silently. When reading aloud, fluent readers read in phrases and add intonation appropriately. Their reading is smooth and has expression.
Sources:
http://www.readingrockets.org/helping/target/fluency#do_kids
https://www.themeasuredmom.com/10-ways-to-improve-reading-fluency/