Scientifically put, IQ indicates the place within a Gaussian normal distribution (i.e., the bell curve of the density function) where the test-taker falls within his or her own age group. Worldwide, most people’s IQs are about 100. This is considered a "normal" or "average" IQ.
The value of 100 has been chosen purely arbitrarily; one could just as well have chosen 50 or 311 for the average IQ score.
After choosing the average value of 100, you can look at subgroups or people whose IQs fall above or below the average.
Looking at many IQ Tests, you can see that IQ values in the general population are distributed approximately - as shown above in the Gaussian bell curve.
The curve also shows that the (arbitrarily chosen) IQ score of 100 effectively divides people into two groups: those with IQ scores equal to or less than 100 and those with IQ scores greater than 100.
You can also see from the curve that 68.2% of people have an IQ between 85 and 115. Only 0.2% of test-takers have an extremely low IQ (an IQ lower than 55), and only 0.2% have extremely high intelligence (an IQ higher than 145).
This means that, statistically, your IQ has a very low chance of being abnormal - but it is not impossible.
Keep in mind that education, age, and how you feel on a particular day, etc., can affect your score, but not to the extent where it is completely different.