beads of sweat
Describes sweat forming large, round drops, often from physical exertion or heat. The expression is somewhat poetic and emphasizes the size and roundness of the sweat drops.
Beads of sweat were forming on his forehead.
The expression combines 玉 (たま, 'ball, jewel') and 汗 (あせ, 'sweat'), literally 'jewel-like sweat', referring to sweat drops that are round and prominent like beads.