Correlation means association - more precisely it is a measure of the extent to which two variables are related.
an increase in one variable tends to be associated with an increase in
the other then this is known as a positive correlation. An example would
be height and weight. Taller people tend to be heavier.
an increase in one variable tends to be associated with a decrease in
the other then this is known as a negative correlation. An example would
be height above sea level and temperature. As you climb the mountain
(increase in height) it gets colder (decrease in temperature).
there is no relationship between two variables this is known as a zero
correlation. For example their is no relationship between the amount of
tea drunk and level of intelligence.