We tell people when we're bored, when we disagree, and when we want them to go on, all without saying a word, or even meaning to let them know how we feel. We adopt certain positions and make subconscious hand gestures; our facial expressions change and our postures relax or become tenser.
Being able to understand these subtle clues about how people are really feeling can be a great benefit to anyone who engages in face-to-face interactions on a daily basis, whether they are receptionists, salespeople or managers, and knowing when to adapt a situation to put your audience at ease or get them interested again can prove an invaluable tool in most circumstances.
Nodding, for example, which we generally accept to mean "yes", can sometimes signal that you disagree or have reservations about what the speaker is saying (this is usually a prolonged nod, meaning, "yes, but…"), while it is also used frequently to let the speaker know that we want them to carry on or that we're listening.
On the other hand when the audience intensely dislikes what the speaker is saying they will cross their arms and legs and straighten up, or rub an eye or eyelid while sitting in a bored position (head turns to the side now and then, head then needs support from a hand and legs and trunk become straighter), letting us know when to change the subject or try a different approach.
In addition to this we can tell when people are getting bored by various cues they probably aren't even aware of, such as twiddling their thumbs, propping their head against their hand, assuming a very slumped posture or picking imaginary lint of their trousers, which is all just their mind's subconscious way of saying that you've been droning on for a while.
Taking note of when people half turn their heads away, fidget, assume a blank expression, crack their finger joints or openly pout or sneer lets us know when rejection is on the cards, allowing us time to change our tactics before it's too late. However, it is arguable that the physical manifestation of these feelings through our body language may prolong and reinforce that particular frame of mind, so picking up on these signs early on is vital.
It is also important to remember that body language needs to be taken into account in context, and that just because the person you are speaking to yawned, it doesn't mean they're not genuinely interested in what you have to say, or that if they bite their nails they're actually nervous.
Read correctly, though, body language can be a huge stepping-stone towards communication excellence.