It could mean there are a lot of [not necessarily impressive] weapons or the [maybe not many] weapons are impressive, or a lot of impressive weapons in the cache.

It means that if someone saw the cache they would say "Wow!"

impressive /ɪmˈprɛsɪv/ adjective adjective: impressive evoking admiration through size, quality, or skill; grand, imposing, or awesome. "an impressive view of the mountains" For this context there was either any weapons and/or some or possibly all were of high quality, such that the cache would impress either the police, or for example a weapons collector/expert. it is always best for provide much more context = more of the full text. Also for writing you could explain and give details as to why the cache is impressive.

It means significant, notable