Fluency is the ability to read a text accurately, at a good pace, and with good expression.
Fluency is tied to comprehension. If a student is reading at a slow pace, and having to sound out many words, the main focus is on decoding, and as a result, comprehension goes down.
To improve fluency, have your child or student read aloud a text that is below his/her instructional level. This can build confidence and show them what it is like to read smoothly. Another good technique is you read a page and your child reads a page. Also, try having your child record him/herself–kids love to hear themselves read!