The audience for papers is other academics in the field. Can result in assumed knowledge. The background section and references are key.
Email the authors, maybe they have source code they can share. Check for supplementals, blog posts, etc. that may have further information too.
Purpose of math notation is to express as much as possible in a compact way. Wikipedia - Glossary of Mathematical Symbols is a helpful resource.
It's not neccessary to understand all the steps in the math. The most important part for implementation is the end formula.
Look for explanations of the math in the text.
Important parts can be terse, papers are information packed and it is easy to overlook crucial information. Key details may be hidden elsewhere in supplementals.
Beware - there could be mistakes!