This one is the lemon plant and what happened to it and what can we do to improve it..?

this can be Castor Semilooper if this is the case then, collect larvae from the plant or remove the affected part. if you want to prevent for future and i always suggest organic method follow the instructions: You will have to make a mixture of paste of neem seed and neem oil 5:2 respective. and apply to affected area specially of early larvae stage. most importantly give good space for birds to reach out to your plant, they will hunt the bigger larvae when you are not around your plant to monitor. Nature is the best solution. kudos. :)