If you flip a coin twice, there are four possible outcomes, two per flip:

First Flip: Heads or Tails
Second Flip: Heads or Tails

So, on two flips, the possible outcomes are:

H-H
H-T
T-H
T-T

Two flips, two possibilities, two times two is four.

Now, visualize each possible selection on the Fantasy Basketball team as a flip of a coin. Two games, four teams, four players per position, so in this case it's like a coin with four sides.

Four power forwards, call them A, B, C, and D
Four small forwards, call them E, F, G, and H

There's 16 possible combinations:

A-E, A-F, A-G, A-H
B-E, B-F, B-G, B-H
C-E, C-F, C-G, C-H
D-E, D-F, D-G, D-H

Four times four. Sixteen.

Bust since there are 6 positions to pick, it's 4 x 4 x 4 x 4 x 4 x 4, or 4096.

Simple enough?