The 300×250 ad unit is an IAB standard ad unit. It has the dimensions of 300 pixels wide by 250 pixels tall.
This ad size is also known as an MPU in the UK. In the US a 300×250 is known as a Med Rec, MREC, or Medium Rectangle.
This ad unit is commonly placed above the fold. It is often used as part of a multi-size ad slot with a 300×600 ad unit. It is generally the best performing ad size, as well as the most expensive/valuable.