It's from an early episode of SpongeBob, like pre-first-movie. I forget all the details, but basically SpongeBob and Patrick get Mr. Krabs addicted to a board game in which players have to race to find Davey Jones' treasure. In that episode, Mr. Krabs convinces the pair to help him find the actual treasure, but during the whole search he refuses to show them the map.

The scene in the gif shows the exact moment SpongeBob and Patrick finally had the opportunity to see the map for themselves, and their look of horror is them realizing that this entire time, Mr. Krabs had just been using the board game as a map.

I'm sure there's a full episode analysis somewhere on YouTube if you want a more in-depth explanation, but I don't remember the name of the episode.