ANN means announcement thread, which is a thread here on bitcointalk that is used to announce an ico (give information etc)
Stakes are basically just a way to divide the rewards of all the bounties. So let's say that we both do a bounty for an ico and the total reward is 1 bitcoin. If I get 7 stakes and you get 3 stakes I will get 0.7 bitcoin and you will get 0.3 bitcoin. The more 'stake' you have in the bounty program the bigger your reward will be.