PolarisingA range is polarised if it is skewed either towards very strong hands or very weak hands. The expression "nuts or nothing" describes the polarisation of a range: If a player is trying to represent a full house by making an over-bet on the river for example, the player either has that full house or he doesn't have any showdown equity and is trying to take the pot with a bluff. Since he wouldn't make this move with a medium-strength hand, his range is referred to as polarised.
By polarising it, the range only contains two opposing types of hands (value hands and air), which makes it prone to exploitation. A player should therefore keep his range as wide as possible, allowing him to represent different hand strengths. This process is called merging your range (merged range).