There are a new special type of bubbles introduced at level 116, locked bubbles with a keyhole in it. They are a part of a chain and locks the chain. Above the chain you have even more bubbles to burst! But they are greyed out until you have unlocked the lock bubble. A keyhole bubble can be any colour and you get rid of it by matching the colour. Levels with a keyhole bubble usually have to played many times, since you need some luck to manage matching the colour of the keyhole bubble.
A sad little fact about lock Bubbles is that they can not be unlocked as singles with a rainbow bubble. :( But If the lock is paired up with a bubble in the same color, shooting a Rainbow bubble at the paired bubble will unlock the lock.