What does critical thinking actually mean in academic work?

Not criticism, and not scepticism about everything — it means evaluating the strength of a claim's support rather than reporting the claim.

What tutors are actually asking for when feedback says "more critical":

Distinguish what a source claims from what it demonstrates. A study may claim broad implications from a narrow finding. Noting that gap is critical analysis.

Interrogate the evidence. How was it gathered? On whom? How large a sample? Could the result be explained another way? Does correlation carry a causal claim it cannot support?

Compare sources against each other rather than describing each in turn. Where do they disagree, and what explains the disagreement — different methods, different populations, different definitions?

Identify assumptions, including the ones the author has not stated and does not appear to have noticed.

Consider what is absent. Which perspectives, populations or counter-examples does the literature not cover?

Reach a judgement, and justify it. This is the step most missing work stops short of. Weighing evidence and declining to conclude is not critical thinking, it is incompleteness.

The descriptive-to-critical shift, concretely. Descriptive writing says what happened, what a source said, and how something works. Critical writing says why it matters, how convincing it is, how it compares, and what follows. A useful diagnostic: count how much of your paragraph is reporting versus evaluating. If the ratio is heavily toward reporting, that is the problem.

What it is not:

Not disagreeing for its own sake. Concluding that a well-supported claim is well supported is a legitimate critical outcome.

Not hedging everything. Endless qualification reads as avoidance rather than rigour.

Not personal opinion unsupported by evidence.

A practical habit: after every sentence reporting something, ask "so what?" and write that answer. The "so what" sentences are where the marks are.