Color splash is a technique to emphasize details- you remove all color from a photo, and then restore original color to a single object, e.g. a green apple on a table. Think of the Girl in the red dress from Schindler’s List.
You can do this in a number of ways with photo editing software– Google has a few suggestions including web browser tools (e.g. fotor), or using a mobile app (e.g. Snapseed). More importantly, how can this be used to draw attention to detail in a photo? What story does it suggest?
Example for "Color Splash":