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A good abstract is a clear summary of the article. A bad abstract is not clear, overly terse (rare) or overly wordy, or even misleading.
Nature gives detailed guidelines for how to write a well crafted abstract. See their guide for an annotated abstract (https://www.nature.com/nature/for-authors/formatting-guide).
In essence you start with a broad introduction, then a more focused introduction, focus in on your question and main result, then move back to a broad perspective of the implications of the result, and then an even broader perspective.
A misleading example is “The health effects of a Roundup-tolerant genetically modified maize […] were studied 2 years in rats. In females, all treated groups died 2–3 times more than controls, and more rapidly. This difference was visible in 3 male groups fed GMOs. […] Females developed large mammary tumors almost always more often than and before controls, […] Males presented 4 times more large palpable tumors than controls which occurred up to 600 days earlier. Biochemistry data confirmed very significant kidney chronic deficiencies; for all treatments and both sexes, 76% of the altered parameters were kidney related. These results can be explained by […] by the overexpression of the transgene in the GMO and its metabolic consequences.” seralini_et_al_2012
The results presented in the paper did not support these statements in the abstract.