The Think () for () Secs block is a Looks block and a Stack block. The block give its sprite a thought bubble with the specified text - the speech bubble stays for the specified amount of seconds. This block is identical to the Say () for () Secs block - except that this block gives a thought bubble, while the other gives a speech bubble.
As this block gives a sort of thought, the block can be used whenever a character thinks something (such as in an animation). Another common use is when unpredictable text must be shown - it is an easy alternative to stamping characters on the screen.