Usually, something you will experience on a aircraft when up in the air due to the plane flying through clouds or a area of unstable air. It consists of light jolts or jerks. That's why you are always asked to keep your seatbelt fastened even when the sign is off as it is sometimes unexpected. It can also be called "Rough Air."
"Ladies and Gentlemen this is your captain speaking, we seem to be experiencing some turbulence, please return to your seat and fasten your seatbelt, Thank You.