To use a really stupid analogy, imagine a cook in an Mafia-involved restaurant, and you're a cop (you're the adversary). There's a whiteboard in the boss' office with a number you want to know.
So you come there every day and say, "Hey, if the lights at (some address) are on, I want a cheese pizza, otherwise I want pasta.". In the beginning he sends his boy to check the light at that address, and the boy takes 20 minutes to go and check, and all that time he has to wait before he can make you the pizza. After a while the boy always says "The lights are on", so after a while the cook would still send the boy, but he would anticipate the answer "the lights are on" and he would start a making pizza. Once in a while (maybe once every 6 months) the boy would return saying the lights are off, so after 20 minutes of making the pizza, he would throw away the pizza and make you pasta.
After doing this a lot, you say, "Hey, if the lights at (some address) are on, I want a pizza with the topping from the box numbered according to the number written on your boss' whiteboard. Otherwise, I want pasta.". You know the lights will be off, but he starts making pizza with some topping you can't tell, but you notice he got a box from the fridge to put it next to his pizza-making table. The boy returns, he throws away the pizza and makes you pasta. Then you ask him, "Hey, I have another order, can I get a pizza with a topping from box 1?". If he goes to the fridge to get that box, then you know the box he picked up earlier (while he was guessing that the boy would say the lights are on) was not box 1, so you ask for another pizza with topping from box 2, etc. But if he doesn't go the fridge, you know he picked up that box previously (because he predicted the boy will tell him the lights will be on), and that's how you figure out what number is written on the boss' whiteboard: when the cook doesn't go to the fridge, it's because the box with that number is already on his table.
The whiteboard is the secret memory area, taking the box from the fridge is reading from some RAM address to cache (if it's already in cache, no need to read from RAM), and I hope the speculative execution is clear.
This analogy describes (very roughly) one of the exploits in Spectre.
So you come there every day and say, "Hey, if the lights at (some address) are on, I want a cheese pizza, otherwise I want pasta.". In the beginning he sends his boy to check the light at that address, and the boy takes 20 minutes to go and check, and all that time he has to wait before he can make you the pizza. After a while the boy always says "The lights are on", so after a while the cook would still send the boy, but he would anticipate the answer "the lights are on" and he would start a making pizza. Once in a while (maybe once every 6 months) the boy would return saying the lights are off, so after 20 minutes of making the pizza, he would throw away the pizza and make you pasta.
After doing this a lot, you say, "Hey, if the lights at (some address) are on, I want a pizza with the topping from the box numbered according to the number written on your boss' whiteboard. Otherwise, I want pasta.". You know the lights will be off, but he starts making pizza with some topping you can't tell, but you notice he got a box from the fridge to put it next to his pizza-making table. The boy returns, he throws away the pizza and makes you pasta. Then you ask him, "Hey, I have another order, can I get a pizza with a topping from box 1?". If he goes to the fridge to get that box, then you know the box he picked up earlier (while he was guessing that the boy would say the lights are on) was not box 1, so you ask for another pizza with topping from box 2, etc. But if he doesn't go the fridge, you know he picked up that box previously (because he predicted the boy will tell him the lights will be on), and that's how you figure out what number is written on the boss' whiteboard: when the cook doesn't go to the fridge, it's because the box with that number is already on his table.
The whiteboard is the secret memory area, taking the box from the fridge is reading from some RAM address to cache (if it's already in cache, no need to read from RAM), and I hope the speculative execution is clear.
This analogy describes (very roughly) one of the exploits in Spectre.