Lento means slow, sitting in a similar tempo range to adagio (roughly 45–60 BPM). While adagio suggests ease and lento suggests slowness, the practical difference between them is subtle and often comes down to character rather than precise metronome markings.
Some composers use lento for passages that are slow and weighty, reserving adagio for slow but flowing music. Barber's Adagio for Strings and Chopin's Lento con gran espressione represent two different qualities of slowness — one sustained and aching, the other deeply personal and intimate. Context and the surrounding music guide the performer's interpretation.