Stand your ground laws are essentially just self-defense laws, and they pertain to the protection of your home, your family and yourself.
According to the Texas Constitution and Statutes, the law requires you to only use force when reasonable to protect against the use of force. There are various situations that will qualify as reasonable.
Criminal activity
In general, criminal activity against you or your property is a valid reason for using force against another person. This would include someone attempting to break into your home or vehicle if you are inside. It is important to note that someone breaking into your vehicle when you are not in the vehicle does not represent a reasonable situation in which you can use force because you are not at risk of direct physical harm.
However, any situation where someone is trying to physically harm you, such as kidnapping or sexual assault, does give you the legal right to protect yourself, which may include using force. Do note, simple assault where the other person does not have a deadly weapon would not allow the use of force on your part. In addition, if you provoke the other person in any way, it removes your right to use force.
Banned instances
You cannot force if the other person is not threatening you with a weapon or something else that could put you at direct risk of physical harm. Someone saying they will hurt you is not enough.
You should note that Texas law does not have a retreat requirement. You do not have to get away from the threat before you use force.