The probability that a coin turns up heads when it is tossed is 1/2. Does that mean if the coin is tossed twice, we will get one heads?

The probability is 1/2 because there are only two
outcomes: heads or tails. Since it is equally likely that either a heads or a tails will
result from a coin flip, this means that the probability of either event is
50%.

You can compare probability to an average. Just
because the average grade in a class is 80% doesn't mean that if you pick two people in
the class both will have a grade of 80%.

You know from
experience that if  you flip a coin twice, sometimes you get tails twice in a row. That
is because each time you flip the coin, the odds remain 1/2; the two flips
are independent of each other. The odds of getting tails twice in a row are 1/2 * 1/2 =
1/4. So 25% of the time you'll get heads twice in a
row.

The odds of flipping a coin 100 times, and getting 100
heads is 1/2^100 = 1/1.2676506 × 1030

But, if you've
already flipped a coin 99 times, and gotten heads each time, then the odds of your next
flip being heads is still just 50:50.

However, if you have
actually flipped a coin 100 times, probably you will have gotten heads about 50 times
and tails about 50 times, because this is the most likely average outcome. But, it
doesn't have to be like that.