The word "lost" has several meanings, depending on its context.
1) If you are referring to an object or something that is misplaced, then lost means not being able to find it or having trouble locating it. For example, "I can't find my keys, they must be lost."
2) In a more emotional sense, the word lost refers to feeling sad, disappointed, or disorientated due to something that has been misplaced or taken away from you. An example of this would be, "After her dog ran away, she felt lost without him by her side."
3) Finally, if used in a more philosophical sense, the word lost can mean being uncertain about one's direction or purpose in life. For instance, "He feels lost in his career and is unsure of what to do next."