A positive correlation means that high values of one variable are associated with high values of the other. Or if you like, the variables increase together.
E.g. As students' confidence increases, so do their grades.
A negative correlation means that high values of one variable are associated with low values of the other or as one variable increases the other decreases. Like a positive correlation, a negative correlation still indicates that some kind of relationship exists.
E.g. As stressful life experiences increase, students' grades decrease.
If there is no correlation between two variables they are said to be uncorrelated. E.g. No relationship between students' grades and if they own a pencil case.
Correlation refers to a measure of how strongly two or more variables are related each other.
ASSOCIATION DOES NOT MEAN CAUSATION.
E.g. There is almost certainly a very high positive correlation between the length of people's right arm and the length of their left arm. But the length of a person's left arm did not determine the length of their right arm. They are both determined by other factors i.e genetics, diet etc.
Correlation coefficient refers to a number between -1 and +1 and states how strong correlation is. If the number is close to +1 then there is a positive correlation. If the number if close to -1 then there is a negative correlation. If the number is close to 0 then the variables are uncorrelated.
Correlational hypotheses predict a relationship between variables, not a difference or effect.