From the Super Mario Wiki
A Boost Block is a block used in New Super Mario Bros. U. Boost Blocks can be used only in multiplayer mode, and to use one, a player must tap the Wii U GamePad screen. Boost Blocks are intended to serve as platforms to help Mario and co. reach the end of the level easily. There are other uses for Boost Blocks, such as placing them above character's heads to prevent falling objects like Thwomps to harm them. If a player steps on a Boost Block while a symbol is shown, the block releases three coins while filling the Boost Star meter, though if a player steps on a block without a symbol on it, the Boost Star meter will disappear, no matter how full the meter was before disappearing. If a player steps on 10 Boost Blocks with a symbol shown in a row, a Boost Star will appear, and the player using the GamePad can tap it to gain Boost Star power.
If a player gets 99 lives, the boost blocks are shrunk to mini versions, making them more difficult to land on. However, it does make it slightly easier to fill the Boost Star meter because the playing-card symbols don't disappear while the mini Boost Block is on-screen.
Boost Block designs