Dimensions in Tableau is defined as a type of data that cannot be measured, which contains qualitative information that cannot be aggregated. As Tableau segregates data into two types, dimensions (which couldn’t be measured) and measures (the one which could be measured).
Excited about learning Tableau, check out our Tableau tutorial to get yourself started. Ans if you are someone who is preparing for Tableau Desktop Qualified Associate certification exam, then I would recommend you to take up the Tableau certification course which offers you industry-standard course instructor-led video modules with guided projects to help you stand through sufficient hands-on experience.
I would suggest you check out our YouTube video on Tableau training for beginners if you have just started out in Tableau.