An example would be when someone puts off housework until they know someone is coming over, and they race around to get the house all tidy before their visitor arrives. It is far easier to give in and be less disciplined when you just have yourself in the picture. When you have a person or people hold you accountable though, you are far more likely to make sure it happens.
An accountability partner is someone you set up a contract with to hold each other accountable for reaching goals. This is a mutual pact and is probably the easiest to set up. The goals do not have to be the same ones, the point is to connect on a regular basis, often once a week, and share with each other how much you have accomplished. The best way to do this is to break up the task into daily chunks and do each one each day so that you are not trying to finish up the project the night before you meet with your accountability partner.
It is often easier to schedule a meeting with one other person than a group of people. You also usually do not have to pay for this method, and most masterminds and coaches are paid programs. You also have more flexibility in how you meet with an accountability partner. You can just call them on the phone, or use an instant messaging program, You could meet for lunch once a week at a restaurant, or at your house.
You may prefer joining a group like a mastermind instead of just a one on one accountability partner. The advantage to having a group is additional pressure on you not to let the whole group down, and the fact when you run across an obstacle, the group can help brainstorm ideas to help you out. Two heads may be better than one, but sometimes multiple heads are much better than just two.
You may find that you want some one on one coaching, or small group coaching instead, to keep you accountable. Maybe you are trying something new and need teaching as well as the connection. One on one coaching tends to be the most expensive option since you are taking up a lot of the time of your coach. Group coaching is becoming far more available because it allows the coach to help more people at the same time, plus gives the advantage of having other people that can help you out with specific issues, like a mastermind. They also usually cost a lot less than one on one coaching so may be more in your budget. Whatever method or methods you use, remember you still have to be disciplined for them to work.