Hey all, I'm an attorney practicing in California. If anyone would like, I would be happy to chat (in a general way; I am not able to give anyone legal advice, and I am not anyone on this forums lawyer) about what is going on in this article.
The extremely short version is:
1) California law has made it "legal" to purchase/sell marijuana in some instances.
2) There is a concept called "Preemption" which ensures that Federal law is the supreme law of the land (under the "Supremacy Clause" of the US Constitution).
3) There is a specific Supreme Court case that says state laws cannot make marijuana legal in spite of federal regulation.
4) There has been some speculation and insinuation (and to some extent, open statements suggesting same by DOJ officials, as referenced in this article) that the Department of Justice would begin to engage in less enforcement of federal marijuana laws in some cases.
5) Federal law enforcement can still enforce federal law, and then send the case to the DOJ for prosecution. The prosecutor can then file a case against you if they want. The US Attorney's office may choose not to prosecute, or may choose to prosecute, it is totally up to them.
Again, happy to write more about preemption/the Supremacy Clause, and how it effects California drug laws if anyone is interested/has specific questions.