Is a simple measure of how much two sizes that are measurable are related to each other.

In finance, correlation is often a measure of how much two completely different markets, sectors or instruments are connected. That is, if a market or instrument goes up, then the positively correlated instrument also goes up.

There is also a term called negative correlation. In other words, there are two markets or instruments that go opposite each other. That is, if an instrument goes up, the negatively correlated instrument / market goes down.

These correlations are something you can find yourself by studying historical charts / price graphs, or by looking at others' technical and fundamental analyzes of different markets and instruments.