Defensives sell things people buy regardless of the economy. Earnings are steadier, dividends are more reliable, and beta against the index is typically below 1, so drawdowns are shallower. The cost is participation: they tend to lag badly when risk-on-risk-off flips to risk-on.
Utilities are the special case. Their steady cash flows make them rate-sensitive, so they can fall in a recession scare if yields rise at the same time.
Example: the index falls 20% and a defensive with a beta of 0.6 falls about 12%. In the following 30% rally it gains about 18%, so over the round trip it finished ahead despite lagging on the way up.
Related: cyclical-stock, sector-rotation, dividend, risk-on-risk-off