The unemployment rate is the share of the labour force that is without work and actively looking for it. Most major central banks (the US Fed most explicitly, via its formal dual mandate) weigh employment alongside inflation when setting policy — a central bank that's comfortable hiking into strong inflation can become far more cautious if unemployment starts climbing at the same time.
What matters most to currency markets is usually the direction of travel rather than the absolute level: a small rise from a multi-decade low reads very differently than the same rise starting from an already-elevated rate.