A child can read every word on the page aloud, smoothly and at speed, and still have very little idea what the sentence meant. Fluency and comprehension are different skills — and telling them apart changes what you do next.
Fluency is the mechanics: decoding the words accurately and quickly enough that the reading sounds natural. Comprehension is what is left over to understand the meaning. When decoding eats all of a child’s attention, there is nothing spare for sense — which is why a fluent-sounding reader can still struggle to answer questions about what they just read.
The quick test is simple: ask your child to tell you what happened. If the words come out smoothly but the retelling is thin, work on comprehension — talking about the text, predicting, questioning. If the reading itself is halting, fluency comes first.