A good way of thinking about a colon is found in the way they are written: colons resemble a pair of car headlamps.
Colons point forwards, revealing more detail. Talking of cars, here's another example:
The car was beautifully painted: it had a union jack on its roof and stripes down each side.
Notice what the colon does: it splits the sentence up into an introductory remark then further details.