Cardinality in databases refers to the complexity or density of data, with high cardinality indicating a large number of unique values or combinations of values. This concept is considered challenging due to its subjective nature and varying interpretations across different contexts. InfluxData DevRel Katy Farmer discusses how to recognize and address high cardinality in databases, highlighting factors such as data organization and relationships between data aspects that influence its existence. Cardinality serves as a measure of data complexity, enabling the understanding of relationships between different data components and facilitating the design of more stable systems.