the root verb is 'chalna' - may mean variety of things depending on the context. few of the meaning may be to walk, to go,to start chalo is imperative form of the verb. a request , an order or advice . so, 'chalo' according to context may mean 'lets go' or 'start walking' 'carry on' or even affirmation like chalo theek hai/koi baat nahin etc. 'chalo' can be better understood in a given context. 'chali' is past feminine form of the same verb. masculine form for the same is 'chalaa'