Accountability in the context of achieving goals, as that is what this website is all about, is where someone else holds you accountable for reaching your goal. There is no clear answer whether sharing your goal is a good thing or not and it should definitely done in a strategic way.
Imagine the following situation; you want to loose weight but your partner is usually buys and prepares the food. Without telling your partner, you decreased your chances of success significantly. In this situation it might be beneficial to tell your partner, however it is not always beneficial to tell someone else.
When you announce your goal to somebody and they affirm it, you feel good [^1]. It makes you feel like you've taken a step towards achieving that goal, even though you haven't done anything yet that gets you closer to your goal. It gives a fake sense of accomplishment which decreases your chances of actually making it happen. Ideally you shouldn't be satisfied until you've actually done the work.
Most of your friends won't care enough to actually keep you accountable. If you do find the right people that can hold you accountable properly, like a coach, then it can increase your chance of succeeding. Find someone that praises the progress you make instead of someone that praise you because of who you are [^2]. Studies have also shown that it is more likely to achieve your goal if you share it with a person you look up to, simply because you care what this person thinks of you [^3].
To make this accountability relationship work even better, you should share written progress reports with your accountability partner [^4]. Assessing the process over the outcome prevents you from having tunnel-vision towards the outcome, losing flexibility to think outside-the-box in situation where that might be beneficial.
[^1] Keep your goals to yourself
[^2] Effects of person versus process praise on student motivation: stability and change in emerging adulthood
[^3] When goals are known: The effects of audience relative status on goal commitment and performance.
[^4] Does Sharing Goals Help or Hurt Your Chances of Success