It's a way of expressing how far you've come along from acheiving a certain goal. For example:
Lets say I had a week to raise $500 for my high school dance club. Two days after the start of the fundraiser, one of my fellow classmates ask, "how are you doing with raising funds?" I could respond, "so far, I've raised $140." You can also say it at the end of a sentence: "I've raised $140 so far."
In this example, it shows that from your final goal, which is ultimately to raise $500 by the end of the week, on the second day, I've only raised $140 of the $500. I'm expressing how close I am to achieving my goal of raising $500.
Hopefully my explanation was helpful.