Glass houses and polythene tunnels increase the heat in the environment that crops are growing in. Reactions happen faster when there is more heat, for example photosynthesis. Given photosynthesis produces energy that the plant needs to grow, if there is more heat there is more growth and so higher yield.
Carbon dioxide is a reactant in photosynthesis. If there is a more than enough carbon dioxide, then every plant will be able to photosynthesise as best as it can. The more photosynthesis the more glucose, the more glucose the more energy, the more energy the more growth. Hence crop yield is increased.