what is the use of group() in this line

[andresrcs]

Even better can you please follow these guidelines about how to properly ask an R related question here?

FAQ: Tips for writing R-related questions

First and foremost, please ask! A core goal of community is to be a friendly place to chat about topics related to data science, R, and RStudio. We know that posting to technical forums can be intimidating. But know that many here would love to see you overcome your inhibition and engage with us. Here are a few tips some folks here think might be helpful. Before you post Check Out R Documentation - R has built in documentation on packages and functions. For example typing ?lm into your R console will open the documentation on the lm function. Search - Be sure to search for the basic keywords of your question with your favorite search engine, Stack Overflow, and R-Help. Stack Overflo…