BLAST is the NCBI/NIH (aka US government) repository for genomic and proteomic sequences, amongst other things. It is where all genome scientists around the world deposit their sequences if they make a discovery. Its main function is to allow comparison of gene sequences and discovery of sequences that match one that you might have come across in your experiment.

What’s a gene sequence? It’s a line of code, made up of any combination of 4 letters in a sequence. BLAST has two sections - nucleotide (BLASTn) and protein (BLASTp). BLASTp deals with amino acid sequences, in just the same way as nucleotide sequences. ¹⁾