What do you want to learn?
Do you want to learn to do statistical analysis with a tool like R and interpret data? Do you want to learn mathematical axioms and theorems about probability and statistics and how to prove them.
Like, mean is an indicator of the central tendency of a sample. Intuitively, it makes sense. But why this particular formula of sum/n?
Depending on who you ask it's not. sum/n is the arithmetic mean. There are also other mean's like the geometric mean and the harmonic mean. Depending on the context different mean's can be used.
It's just a convention that one usually means the arithmetic mean if one says mean. It has the advantage that's relatively easy to calculate by hand and therefore people liked it. It's not complicated to do mathematical proofs with it.
One of the key reasons why few people use robust statistics is that the math is more complicated.
This is even a political issue. In soviet Russia the usage of the arithmetic mean was very looked down upon. People were supposed to use the median. Sometimes that communist thought got in the way for cases where the arithmetic mean was really appropriate.
When it comes to learning notation. Anki is quite good.
In soviet Russia the usage of the arithmetic mean was very looked down upon. People were supposed to use the median.
That, um, sounds like an urban legend to me. Links?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.