Family Resemblance: Things are defined by overlapping similarities, not a single common essence, much like traits in a family.
We (and early Western philosophy) often think everything has an essence. Similar to Essentialism or Plato's idealism, we believed that, for any object, there must be a set of essential features that all members of that category share. The more precise we can define the essence, the closer we are to the truth.
However, how do you define terms like "Game"? More specifically, what's a characteristic common to all games? Competitiveness? Basketball games are competitive, but make-believe plays aren't. Involve physical activity? Board games don't seem to fit. Require skills? There's no skill required for kids to play in the sandbox. Although we can't find one thing all games share, we still recognize games as games.
Ludwig Wittgenstein argues that, instead of a common essence, things may be connected by a series of overlapping similarities, where no feature is common to all things. Wittgenstein called this family resemblances, where we categorize stuff like how we recognize family members. Some family members may share the same eye color, and some may share the same temperament, but we can't find the same set of features that everyone has.
Therefore, we shouldn't pursue the ultimate essence of things as the way to get closer to the truth because the members of a conceptual category (like games) may share overlapping similarities without all having the same features.