I understand, the $ can be confusing when you're first learning R.
The $ is used to extract specific columns (variables) from a data frame. A data frame is the structure that the read.csv function returns, which contains the imported data. It is like a spreadsheet, with rows and columns.
So in this code:
data()$variable

data() refers to the data frame of imported data
$variable extracts just the variable column from the data frame

This is done to plot or analyze a specific column of the data, rather than the whole data frame at once.
So the overall flow of the code is:

Import CSV data (into a data frame)
Extract columns of interest from the data frame
Analyze and visualize the data using the extracted columns
Present the analysis and visualizations in the dashboard

Does that help clarify the use of $ and the process? Let me know if you have any other questions!
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