What is the difference between は and が, subject marking particles?
See the two sentences below:
ちち は チョコレート を たべました。
ちち が チョコレート を たべました。
Both of them mean ‘My father ate chocolates.’
The first sentence with は basically just gives you the fact that he ate them.
The second sentence with が implies that the person who ate them is him. Not me, not you, that is him.
There are much more things about は and が, but this is one of the most important aspects you should know.