Q1 means that that specific journal ranking appears in the top quartile (Q1). There are 4 quartiles from Q1-Q4. To ensure you refer to the best journals for your research topic, you should aim for Q1 or Q2. You can find out the rankings of journals on the website http://scimagojr.com
Bear in mind that depending on the nature of the article/subject matter a journal may present itself in several different quartilesand this varies over time. When I do research I always check this first (aim for Q1) and then the author to see if they are an expert in the field.