MC1R
MC1R melanocortin 1 receptor — the molecular target that 1 compounds in the wiki act on.
In one line: The skin's pigment receptor — activating it produces a tan.
MC1R is the receptor on skin cells that controls pigment. Activate it and your skin produces more melanin — a tan. The compound melanotan works by pressing this receptor to tan skin with less sun exposure.
The concern is real: it also darkens existing moles, and its long-term safety (including melanoma risk) is unknown. So while it does produce a tan, it sits firmly in the "unresolved safety questions" category.