A hot block is a term used to refer to a block with fire underneath it. Generally, it refers to ones in a group of blocks, where Chip has to select blocks to either push or to use, and the wrong choice will cause his death. Examples include the south-east section of Block N Roll, the block area of Lesson 4 where the idea was introduced, and throughout Special.
Although there may be fire under a block, blocks will not be destroyed if fire is underneath them. They can start, be pushed onto, or even reveal fire hidden under the floor.