Particle - は - Topic marker
The particle は follows the topic the speaker wants to talk about. Therefore, は is often called topic marking particle.
It is often used in a sentence pattern like, AはBです, which means A is B.
Note that the topic is defined for the context, and not just for the sentence. This means that all following sentences use the same topic. 猫はかわいいです. 怖いです (The cat is cute, the cat is scary)
Often a topic is not defined at all. The default topic of a sentence is I (the speaker of the sentence). For example: さくらです (I am sakura).
The last train will go away at 9 pm
Today I came by train
I was in the mountains.
Is that a recent photo?
It depends on the context.
How many close friends do you have?
When I woke up, I was sad.
I walk every morning.