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.
Incites Journal Citation Reports (JCR)
Probably the best known resource to assess journal impact (and the one that's been around the longest) is Incites Journal Citation Reports (JCR). JCR allows authors to look up impact factors by individual title or to compare impact factors of journals within a subject category. In addition to providing journal impact factors, JCR also reports the Eigenfactor Score of a journal as well as its Article Influence Score.
Scopus Journal Metrics
The Scopus database provides access to CiteScore which provides a means of assessing the impact for a specific journal and/or to compare rankings of journals within a subject discipline. Scopus also allows authors to identify the Source Normalized Impact Per Paper (SNIP), SCImago Journal Rank (SJR) and Impact Per Publication (IPP) of a journal. The journal analyzer supports comparing these metrics for up to ten journals. Click on Sources to access journal metrics within Scopus.
Google Scholar Metrics
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. That being the case, 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.