A condition is sargable (Search ARGument ABLE) if the engine can rely on an index to evaluate it. Applying a function to the indexed column (WHERE date_trunc('day', ts) = ...), a calculation or a LIKE '%x' prevents it. You then rewrite the condition to isolate the column, or create an expression index.
Example
WHERE ts >= '2025-03-01' AND ts < '2025-03-02' -- sargable