"Subject to" can mean different things in different context. Here are two common examples: "Your free lesson is subject to the availability of the teacher" => Your free lesson will only happen if the teacher is available. In this example "subject to" is used to refer to something being conditional on something else. === "The criminal was subject to interrogation by the police" => The criminal was forced to endure being questioned by the police. In this example "subject to" is used to show that the criminal is going to encounter something unpleasant or harmful. Usually we don't use "subject to" to refer to something nice or pleasant.

I like the online Collins dictionary as a general purpose dictionary for English learners. It includes word phrases such "subject to" (which has a different meaning from "subject"). "subject to" entry https://www.collinsdictionary.com/dictionary/english/subject-to [excerpt] frequency - among the most common 4000 words subject to (preposition) meaning - under the condition that example - we accept, subject to her agreement

I need to take this subject to complete my degree. It means I need to pass a certain course to graduate.