The points to one specific thing that both speaker and listener already know. I saw a dog in the yard, then I fed the dog, since the first mention identifies it. Some things need no first mention: the Prime Minister or the sun are definite through shared context.
A tiny dictionary entry, in action
The fits nouns that are one of a kind or already known to both speaker and listener. The sun rose late today works because there is only one sun in our sky. The water in this bottle is cold works because the bottle pins down exactly which water you mean.
Once you have named something, later mentions usually take the as well. I saw a dog in the yard. I fed the dog. The first mention uses a because the listener does not yet know which dog. The second uses the because now the dog is identified.
When not to reach for it
The nearest mix-up is between the for a specific group and plain plurals for all of a kind. Say Children learn quickly when you mean children in general. Say The children in my class learn quickly when you mean one known group.
Abstract nouns work the same way. Love is important talks about love in general. The love I felt was sudden points to one particular love, so the fits there.