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What is the significance of k-mer length in sequence analysis?

I'm getting started in biology and bioinformatics with sequencing the SARS-Cov-2 or Coronavirus genome. I'm interested in this code which identifies k-mers in the genome:

from skbio import DNA 
DNA(str(seq.seq)).kmer_frequencies(5)
kmers = DNA(str(seq1.seq)).kmer_frequencies(5)
kmers
{'ATTAA': 60, 'TTAAA': 95, 'TAAAG': 64, 'AAAGG': 45, 'AAGGT': 46, 'AGGTT': 51, 'GGTTT': 55, 
'GTTTA': 69, 'TTTAT': 75,...}

The author of the code wants to use k-mer breakdowns to vectorize the data for machine learning purposes. What I'm curious about is why the k-mer in this case is five nucleotides? What factors should go into my selection of the length of the k-mer for analysis?