When looking at the relationship between two things, correlation and causation are concepts that are often confused. Correlation refers to things that appear to have some form of consistent relationship, i.e. one item increases at the same time the other increases also. Based on observations many make the mistake of assuming that correlation can imply causation, that the increase in one item is increasing the other.
The distinction between the two is very important as when some see a correlated relationship they may make decisions based on what they are seeing. This may work out well for them if they're lucky and the correlation continues, but as the one event doesn't actually cause the other it can lead to poor decision making.

If there is an actual causation relationship then making a decision to do something based on seeing the other event take place is actually good decision making.

From an investment perspective this can be important. If an investor thinks they see a causation relationship, i.e. a firm's stock price goes up a few days after every positive employment report, and it is actually a correlation they could lose a lot of money. Say the next time they see a positive employment report they put all their portfolio into futures on this stock and this time the correlation doesn't occur they could lose a great deal of their investment. Understanding relationships, and putting the research into understanding what you think you see, is important.