on hand idiom (UK also to hand)
C2 在跟前，在旁边
near to someone or something, and ready to help or be used if necessary
A 1,200-strong military force will be on hand to monitor the ceasefire. 将有一支1200人的部队在现场监督停火。
For those of you who don't have an atlas to hand, Newcastle is a city in the northeast of England. 谨向手头没有地图册的人说明，纽卡斯尔是英格兰东北部的一座城市。