For some of you this information might seem redundant, but it's crucial that we are speaking the same language.
Your 6th string is the lowest and closest to you. It gets called the "bottom" because it is the lowest pitch. The strings are numbered sequentially from there 6 to 1.
We'll play the 3 bass notes with our thumb. When I am referencing a "low" bass, I'm talking about the 6th string. When I reference a "middle" bass, I'm talking about the 5th string, and when I am referencing a "high" bass it's the 4th string.
Fingers are represented by "T" for thumb, "1" for index finger, "2" for the middle finger, "3" for the ring finger, and "4" for the pinky (which we won't be using today).