Check out example codes for "convert gene id to gene symbol in R". It will help you in understanding the concepts better.

Code Example 1

library("biomaRt")
ensembl = useMart("ensembl",dataset="hsapiens_gene_ensembl")
getBM(attributes='hgnc_symbol', 
      filters = 'ensembl_gene_id', 
      values = ensemblsIDS, 
      mart = ensembl)

Code Example 2

library("org.Hs.eg.db") # remember to install it if you don't have it already
symbols <- mapIds(org.Hs.eg.db, keys = ensemblsIDS, keytype = "ENSEMBL", column="SYMBOL")

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