When you're sociable, you're willing to talk and be friendly with other people. At a party, it's a good idea to be sociable and make new friends rather than standing off in a corner by yourself.
Sociable comes from the Latin sociabilis meaning "close, intimate." When you are sociable with others, you invite them to get close to you by being friendly and pleasant. When a place or group of people is friendly and inviting, it can be described as sociable. Certain small towns are known to be friendly and sociable.