The x-axis is dependent on and represents your input list of ranked genes - exactly how you have ordered the list is important for the interpretation of the plot's overall result. For example, are the most statistically significant genes on the left or right?; or, are those with largest fold-change on the left or right?
The black vertical bars along the x-axis are those genes from your input list that feature in the gene-set / signature that is being enriched.
The green line is the enrichment of this gene-set / signature, and it can be positive or negative.