A journal is considered to have a high impact if the articles it publishes are frequently cited. A number of resources are available that rank/score journals by their impact/influence.
The impact factors/metrics within some disciplines will be higher than others, e.g. journals from medicine, biochemistry, cell biology are among the journals with the highest JCR impact factors while journals from economics, linguistics, and nursing are among those with lower impact factors. This is not a reflection of the quality of the journals in one discipline compared to another, but of the publication and citation practices of a discipline as well as the publications that are being analyzed by a particular metrics tool. When assessing the impact of a journal, no matter which metric you use, it makes sense to compare a journal to others within the same discipline or in other disciplines of relevance to the topic of your paper.