I'm guessing:
You do not understand regular expressions, so a lot of sed examples do not make sense.
try this to get started understanding sed -
sed, a stream editor

There is a section on learning about regular expressions. perl uses regular expressions as well. so does awk. Along with grep (uses regex as well) these utilities are probably the most commonly used tools on unix systems. So learning a little bit about regex ( regular expresssions) is very important.