Agreeableness means being nice and friendly to people. Imagine you are playing with your friends and one of them wants to play a different game than what you were playing. If you say okay and change the game to make your friend happy, that is being agreeable.
Being agreeable means you try to make other people feel good and comfortable around you. You might say nice things to people like "That's a cool toy" or "I like your shirt". Being agreeable is all about being kind to others and making them feel good.