A tango practica is an informal class, i.e. no structured group lesson.  Dancers are free to work on whatever interests them with an experienced instructor handy if they need advice.  

Beginners are always welcome and will receive a micro-lesson at some point during the practica.  More experienced dancers typically invite the beginner to dance, and will give helpful suggestions.  However beginners are advised to also take either structured group classes, or private lessons (or both) if they are serious about developing a solid foundation.