锄草 means 'to weed (using a hoe)'. As a separable verb (离合词), the object 草 can split from the verb 锄 when measure words, durations, or complements are inserted — for example, 锄了一次草 ('weeded once') or 锄完草了 ('finished weeding'). You can also say 用锄头锄草 to emphasize the tool. It's a rural agricultural term, specific to hoeing as opposed to hand-pulling weeds (拔草).