They leave long trails behind them, tracks you might say as they move from one point to another.
他们在身后留下了长长的踪迹,也许你会说是他们从一个点移向另一点时留下的.
tracks 是 trails 的同位语,you might say ... 是 tracks 的定从,as 在这里的意思是“当/随着”.
原句基本上就这意思--> you might say (that) they leave long trails/tracks as they move from one point to another.