The basic principles of grammar aren't that difficult.

Literal

Grammar [possessive-の] basic-principles [topic-は] that-much difficult [topic-は] not.

それほど…ない means 'not that much' or 'not so.' The double は — one marking the topic (基本原則) and one inside the negative predicate (難しくはない) — is natural in negative statements, where は softens or emphasizes the negation.