"Cost prohibitive" describes something that is difficult to obtain because of its cost. In this phrase, the word "prohibitive" is used as an adjective, and it refers to the act of preventing the use or purchase of something. The word "cost" clarifies that the prohibitive nature is due to the cost of the item.
If something is cost prohibitive, it has such a high price that someone struggles to be able to afford it. The phrase "cost prohibitive" is subjective, and an item that may be affordable to one person may be considered cost prohibitive to another person. For instance, if a child wants to play soccer, but his parents cannot afford it, the parents could say that soccer was cost prohibitive. However, another set of parents may claim that soccer is very affordable.
In some cases, governments may add taxes to make an item cost prohibitive to curb its usage. For example, the government raises taxes on cigarettes so the product is cost prohibitive for many people. Theoretically, this makes it less likely for people to consume the product. Cigarette taxes and similar consumption taxes discourage or prohibit use of a product based on cost rather than directly prohibiting the use of the item.