When something "depreciates in value", it means that its value decreases. For example, most cars depreciate in value over time. A new car costs a lot more than an older car, and the older the car is, the less it's worth.
This is a slightly technical-sounding term. A simpler way to describe this is "goes down in value":
Cars go down in value the longer you own them, so it makes more sense to buy a used one.
The opposite of "depreciate" is "appreciate".