Vague checklist items lead to confusion and errors. Each item should tell you exactly what to do.
Instead of "Check the system," write "Verify database connection responds within 2 seconds."
Instead of "Review the document," write "Confirm all section headings follow title case."
The more specific your items, the less room for interpretation. This means different people using your checklist will do the same thing consistently.
Specificity also makes it clear when an item is complete. You should be able to answer with a definitive yes or no.
If you find yourself unsure whether an item is checked, it's probably not specific enough.
References
Scriven, M. (2000). The logic and methodology of checklists.