Siddhartha Gautama was the man whose teachings founded Buddhism. Buddha is his honorary title, but in Buddhism people who experience enlightenment and awakening as per his teaching are also referred to as Buddha. Buddha achieved the purity and perfection of his spirit by his own efforts and thus achieved an unlimited development of all his potential.
The image of the first Buddha is, therefore, a symbol for the search for one’s own enlightenment and is therefore also popular as a meaningful tattoo motif.