Sexual assault is sexual contact you don't consent to.  It can include actions from groping through to rape. It would include actions taken when someone is not in a position to decide or to say whether they consent to what is happening, for example because they are drunk or drugged.  It could involve threats, blackmail, physical force or other types of influencing or persuasion.
Sexual assault can happen to anyone and can be done by anyone.  Sexual assault can be carried out by someone known to you as well as by a stranger. The fact that there had previously been a social or sexual relationship, or that a relationship is still ongoing between two people does not mean consent is not necessary for future sexual activities to occur. You can be in a relationship and still experience sexual assault.  You also need consent to change the nature of the activity, for example to remove a condom or to use force.