A Bear market is when Stock Markets decline in value over a sustained period.
You often hear people talk about a Bull Market or a Bear Market, and these are similar but opposite terms. A Bear Market is when Sock Market prices decline by around 20% or more over a sustained period.
This can happen when there are problems with an economy, perhaps during a recession. The Global Financial Crisis of the late 2000s or the .com bubble bursting in the early 2000s are examples of this.
But why is it called a Bear Market? Well when a bear attacks you it swipes downwards with it paws. So the bear attacking downwards reflects the Stock Market heading downwards. This is of course the opposite of a Bull Market, where a bull attacks upwards with its horns!