To be "in a rut" is an idiom that means to be stuck doing the same thing over and over again with little or no enjoyment or opportunity for progress. Someone with a suffering social life might feel bored with the nightly routine of eating dinner and watching television alone, but be too afraid to reach out to other people to try and make new friends. This person could be described as stuck in a rut, because he or she is unhappy with the current situation but keeps repeating the same actions without any attempts to break free of the cycle. The issues holding the person back from making progress can be nearly anything, from fear and other psychological issues, as with the previous example, to a lack of time, money, and other resources.
This phrase can be used both by outsiders to a situation describing another individual or as an admission from the individual involved. For example, an ambitious employee who has successfully advanced through a company may look at less driven coworkers who continue to do the same job every day with no hope for promotion as being stuck in a rut. Similarly, an individual who hates his or her job but feels overwhelmed by the poor choices for other jobs may decline to look elsewhere and instead complain that he or she is stuck in a rut.
While many people seem to complain of being unhappy in a thankless job environment, the idiom can also be used to refer to anything from overall life patterns to small, seemingly inconsequential things. For instance, while one individual feels trapped in a repeating cycle of abusive relationships over a span of many years, another may simply feel stuck eating the same boring foods every night for dinner because of a lack of cooking creativity. Both individuals in this case clearly fit the definition of being unhappy with the current situation and unable to escape from it, however the severity of the situations is hugely different. While the person bored with dinner can break out of his or her rut with the help of a new cookbook, the other person might need years of help from family, friends, and psychologists to free him- or herself from the rut.
The idiom was likely derived from the days when wooden wheels cut deep ruts, or grooves, in the dirt roads in frequently traveled areas. If a wheel got stuck in one of the previously carved ruts, it became difficult to steer off of the previously traveled path and go a different direction. It is a small step from this literal definition of being trapped following the same, well-traveled path to the more metaphorical meaning of the modern idiom.