Any witness called to testify must have "competence" to testify. This simply means that they are legally permitted to give evidence. Certain classes of people are not considered "competent" to testify due to their personal or professional characteristics. An infant, for example, is never competent to testify.
The compellability of a witness refers to the power to force a competent witness to testify even if it's against their will. If the witness is not willing to participate voluntarily they must be "compellable" before a subpoena can be issued.