Incoherent thoughts don't follow each other logically. Incoherent speech is mumbled or jumbled. Incoherent means that something is difficult to understand because it's not holding together.
A lot of people use incoherent to mean unintelligible, which is a perfectly fine usage. But it specifically means unintelligible due to a lack of cohesion, or sticking together. An incoherent argument may sound something like this. "I deserve to go to the dance because it is the second Tuesday of the month and my feet are a size ten." The reasons do not follow each other logically and to not even relate. It's an incoherent mess.