An ideal is a defining Idea or moral stance that overwhelms other beliefs you might have.
How to Choose a good Ideal.
Think of your personal morals or opinions, then consider how those might impact how you will play the game. Use this information to form the Ideals of your character.
A character made up of only numbers is boring. Your characteristics are what defines your character and your ideal is a very important part of this. It is what your character idealises and works towards making a reality. A good choice of an ideal will also help your DM make a compelling story that your character is invested in.