The Exponential Time Hypothesis (ETH) posits that there is no algorithm that can solve all instances of the 3-SAT problem in subexponential time, specifically, it suggests that the time complexity cannot be less than 2^o(n) where n is the number of variables. This hypothesis is significant in computational complexity theory as it implies that certain problems are inherently difficult to solve, providing a foundational assumption for classifying the complexity of various computational problems.