I can say only what I think they mean by them. Christians can be set right with God and how they can work towards holiness. By meat I think they mean a rigorous examination of the subtle doctrines and mysteries of theology. They seem to think that each diet has its place. Milk is all right for anybody, butit is especially for people whose mental grasp of Christianity is shallow.
Meat is for people with better training and more inquiring minds.
Milk is likely to soften disagreements between Christians and to play down distinctions between doctrine. Meat, on the other hand, is apt to emphasize these differences and to trace their implications through obscure issues. This, I think is what people mean by milk and meat. This milk and meat talk comes from the Bible. You need milk, being still an infant,is not acqainted with the teaching of righteousness.