To thicken means to become denser, more concentrated, or less watery. When you make hot chocolate and it starts to thicken as it heats up, the liquid becomes richer and coats your spoon instead of running right off. When you add flour to gravy, you're thickening it so it clings to your mashed potatoes instead of pooling around them.

Thickening happens in cooking all the time. Jam thickens as it boils and the water evaporates. Pudding thickens as cornstarch absorbs liquid and swells. Even blood thickens into a scab when you get a cut, forming a protective seal over the wound.

The word works beyond cooking too. Fog can thicken on a cold morning, getting so dense you can barely see through it. A mystery plot thickens when new clues make the story more complicated and interesting. When someone says “the plot thickens,” they mean a situation just got more complex or mysterious, like when your missing homework turns up in someone else's backpack and now you need to figure out how it got there.