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

ensembl = useMart("ensembl",dataset="hsapiens_gene_ensembl")
      filters = 'ensembl_gene_id', 
      values = ensemblsIDS, 
      mart = ensembl)

Code Example 2

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

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