A small light tap is one of the easiest and most basic embellishments, and is a nice way for the follower to mark and express the double-time ('quick, quick') - or the 'and' count in between the beats - while the leader is leading a step on the whole beat ('slow'). A tap can be done in the middle of virtually any step - back, side, forward, ocho.
At the mid-point of the step, as your free foot brushes past your standing foot, tap your free foot against the ground. Do it with just the ball of the foot, with the foot tipped slightly towards the metatarsal/big toe edge (this poised foot placement is the common aesthetic of tango).
Make sure you still arrive into the next step led by the leader on the whole beat. Also be sure that after you tap you keep stretching your leg back straight into the next step. Do not undercut yourself and step shorter than the leader.
Early tango was known as tango criollo, or simply tango. Today, there are many tango dance styles, including Argentine Tango, Uruguayan Tango, Ballroom tango (American and International styles), Finnish tango and vintage tangos. What many consider to be the authentic tango is that closest to that originally danced in Argentina and Uruguay, though other types of tango have developed into mature dances in their own right.