The bottleneck theory suggests that individuals have a limited amount of attentional resources that they can use at one time. Therefore, information and stimuli are 'filtered' somehow so that only the most salient and important information is perceived. This theory was proposed by Broadbent in 1958. Picture a large bottle filled with sand that is turned upside down. The bottleneck restricts the flow of sand so that it slowly pours out instead of coming out all at once. Stimuli in our environment is the same way - if we perceived every visual, auditory, olfactory, and tactile sense all at once all of the time our brains would be overflowing. I bet you didn't notice the feeling of the chair (or ground or bed, wherever you are) on your back until you read this sentence.
Now you can feel it because you are actively thinking of it. So there is a suggestion there is a mechanism of sorts that limits the amount of information we pay attention to. How does the bottleneck work? What is filtered out? Broadbent proposed early selection - that physical characteristics of messages are used to select one message for further processing and all others are lost. In 1963 Deutsch & Deutsch proposed that all messages get through, but that only one response can be made which was coined late selection.
Then in 1964 Treisman proposed attenuation which suggests that physical characteristics are used to select one message for full processing and other messages are given partial processing. Research suggests that attenuation is most likely the most influential theory as to the filtering process of the bottleneck.