Loser is a term that originally referred to someone who had lost at something, such as at a game. Its meaning has shifted sometime in the last 30 years to subjectively and pejoratively include anyone who would "lose" at life itself. Some consider those to be losers who, despite decent economic conditions, cannot find work, manage their money, or maintain meaningful relationships. A loser could be a person with little or no ambition for the future, or someone that is always at loss, has accomplished little relative to their stage in life, or simply "has no life."
The term itself can range from offensive to less than serious. People may consider others losers because of the degree to which they have ruined their lives. They may consider friends losers because of the mistakes they have made, or the boring lives they lead. Examples would be people who fail to prioritise effectively in their lives, putting things which do not benefit them or make them happy above those things which do. People may also consider themselves losers because they believe they are not living the lives they consider desirable.
To avoid negative connotations, one who just misses winning at something is often called the runner-up instead of the loser.