What's the difference in stage 1 and stage 2 in baby food? my doctor told me to give him Prune baby…

What's the difference in stage 1 and stage 2 in baby food? my doctor told me to give him Prune baby sauce but I don't find it anywhere in stage one i only find it in stage 2 and it's prune mixed with other fruits -_- .... (he's constipated) oh and he's only 4 month and I know stage 2 starts from 6 months but would it harm him or something