The cabecco, as explained below, is the traditional way you invite someone to dance in tango. Many new dancers can find it a bit intimidating, but it initially arose to make asking someone to dance less awkward. If you don't like the cabeceo, you can always ask people to dance the regular way by going up to them.
The cabeceo is a nod of the head that is used to ask and accept offers to dance. Its main benefit is that you can ask someone (if they're looking your way) to dance even if you're across the room which saves you the time and effort of trying to fight your way through a sea of chairs, people, and tables. Its also to help avoid embarrassment. If you went all the way over to the other side of the room to ask someone to dance and they declined (maybe their feet really hurt and they need a break), it can be a long walk back across the room in search of someone else to ask.
The following steps were adapted from a great website called TangoChaos.
1. Identify the music of the tanda, and then immediately begin to stare intently at your first choice for that type of music.
2. Do not take you eyes off that person. (If you have a history, the rest is easy, because he or she will probably already be looking back when they hear the music).
3. If no eye contact is returned, wait a bit. If you sense the person is aware of you, but is looking elsewhere, immediately switch your stare to choice number two, and repeat the process.
4. If eye contact is made, any sign of recognition will work. This is usually nothing more than a glance of a second or two, or maybe a slight nod, or a cutting of the eyes toward the floor.
5. If you happen to make eye contact by mistake with someone you don’t want to dance with, show no reaction at all, and look away quickly.