Rich black
Black is not always just black. Pure black from a single ink (K 100) quickly looks somewhat grey and uneven on large areas. For a deep, saturated black you therefore mix a rich black from several CMYK colours, for example C 40 / M 30 / Y 30 / K 100.
Correct use matters: large areas benefit from rich black, while fine text and thin lines should be set in pure K 100. That keeps them crisp and prevents “flickering” caused by slightly misaligned inks.