A high impact journal is, for simplicity, a journal that gets cited a lot. Nature, Science, and Cell come to mind. An impact factor is basically an average of how often an article published in that journal gets cited. However, you should keep in mind that not all journals with high impact factors are necessarily good or prestigious or hard to get published in. For example, some journals only publish reviews. Since reviews get cited a lot more often than research articles, these journals tend to have impact factors skewed in the positive direction.
Also, this is not directly relevant to your question, but high-impact journals like Nature and Science also have some of the highest rates of retraction. To get published in these journals, your work has to be truly significant and impact your field. Some PIs tend to aggrandize their results and over-extrapolate, leading to interpretations that are later retracted. Keep these ideas in mind as you evaluate journals.