The curve plots Treasury yields from three months out to thirty years. Normally it slopes upward, because lending for longer carries more uncertainty and demands more compensation.
An inversion, with short yields above long ones, says the market expects rates to be cut, which usually means it expects the economy to weaken. The 10-year minus 2-year spread and the 10-year minus 3-month spread are the two most watched versions, and both preceded every US recession of the past half century.
The lead time is long and unhelpful for timing, often a year or more, and the un-inversion rather than the inversion has tended to mark the point at which trouble actually arrives.