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There is a rabbinic requirement to recline when we eat matza and drink wine at the Seder, as part of acting as if we have just been freed from Egyptian bondage. How does reclining express freedom? Someone who is 'at readiness' sits up straight, to be able to spring into action when summoned to fulfill his duty. In contrast, someone who has been freed from all obligations can casually lean back, allowing all of one's muscles to relax.
There was a time when people normally sat on pillows placed on the ground, which meant that sitting up straight indeed required effort. In such circumstances, reclining – the position between sitting and lying down, in which the entire body stretches out on a couch or pillows – was very comfortable and demonstrated a sense of freedom. Today though, when we normally sit on chairs, reclining at the Seder involves sliding one’s bottom forward to the middle of the seat, leaning back against the chair back, and tilting left. (Ideally, the chair should be comfortable and have armrests.) Why recline to the left? First, it is easier to eat this way, as it frees up the right hand, with which most people eat. Additionally, in the past, some people were concerned that reclining to the right might cause food to go down the trachea instead of the esophagus, which could lead to choking.
The requirement of reclining applies when fulfilling the mitzva of eating matza, and while drinking the four cups of wine. It is commendable to recline during the rest of the meal as well, if comfortable. We do not recline while eating the maror, because maror represents slavery. One should not recline during Birkat Ha-mazon, as it must be recited with awe and reverence. Likewise, it is customary to refrain from reclining while reading Maggid, to make it easier to fully concentrate and treat it with the seriousness it deserves.
