If you have been arrested, if the officer intends to interrogate (question) you, the officer is required to inform you of your Miranda rights.  When "in custody", the law requires the officer to place you on notice that you have the right to remain silent, you have the right to have an attorney present with you during questioning, and any statements you make may be used against you in court.  If you are "in custody" and you are questioned without having first been advised of  your Miranda rights, then an attorney can motion for the Court to suppress any of your "in custody" statements.  On the other hand, if you are NOT "in custody" and you make statements, those statements will most likely be admissible in court.