In 0-based coordinate systems, the start of the first position in a sequence is counted as 0, not 1. For example, in a 0-based system, the first nucleotide of a chromosome is at position 0, the second at position 1, and so on. This is different from 1-based coordinate systems, where the first nucleotide is at position 1. Many bioinformatics tools, including the UCSC Genome Browser, use 0-based coordinates for specifying genomic locations. You can find more information here.