It depends on how your ui is set up - for example, if you're using fluidPage, you could add a new fluidRow below the content area for your plot, and place your explanatory text inside it.
If you've made an attempt that isn't quite working, you may consider creating a shiny reprex (reproducible example) - this is a minimal working example that will allow us all to look at the same code and data, which will help you get more targeted help for your question. Please see the FAQ for shiny debugging and reprexes for a good guide!