Q1, Q2, etc. indicates the impact quartile that a journal is in for a particular field of research. A journal may be ranked as very high impact in one area (Q1) and lower impact for another (Q2 or Q3), depending on the performance of other journals which also publish in that field.
You can find the Q rating for a journal in Scimago (free website) - search for the journal by name or ISSN. Scroll to the bottom of the the journal information page to see the impact quartile for its field/s listed over time.
