In the simplest of terms, a chip leader in poker is the person with the largest number of chips in their stack relative to other players. The term is usually used in tournaments, although you’ll sometimes hear it at cash game tables as well.
For example, in a 100-person tournament, one player has a stack of 1,000,000 chips, while everybody else has less than that. That person is the chip leader for the entire tournament.
The term can also be applied to a single table, so someone can be referred to as the chip leader at a particular table since they have more chips than any other player seated at that table.
Being a chip leader in a poker tournament is nice, but it is one of those things that constantly change. It may give you a short sense of accomplishment but don’t give it too much attention, especially with many players still left in the field.