Why is your xbox 360 is flashing red

The red flashing lights are a sign of a fault with your console.
You need to give more information than the 360 is flashing red in order to determine the exact fault. How many red lights are there and what corners are they in? If it is 3 red lights, the most common fault, then here is a little explanation:

firstly if you get the 3 red lights then Microsoft reapair your console for free so long as you purchased it no more than 3 years before you send it back for repair. The 3 red lights are a sign of overheating, and to go into more detail this is what happens:

The 360 had a manufacturing fault or the air vents were covered and so the 360 overheated, and got so hot that the solder balls on the CPU or gpu chip melted, and due to the fact the the 360 motherboard is able to flex a little, when the 360 was turned off and cooled down these solder balls solidified again, however they solidified in the wrong place or in two halfs etc. and so the 360 shows the 3 red lights to indicate this fault.

James