wat dos dog ehistle meann

A dog whistle in this context is saying or implying something that could be easily denied, but was intended for a certain specific audience to understand, like how a dog hears the whistle but the owner can't.

So I could say "at around 14:00 I helped an 88 year old man at the protests, we should all start doing the same"

That alone is just a statement about an old man at the protests, the dog whistle is "I was counter protesting with my Nazi friends and we need more of us involved". Since 14 and 88 are Nazi codes, they're put in as unnecessary details that wouldn't normally stand out or really be mentioned in a normal conversation.

A dog whistle is something most people won't hear/see, but those in the know will. Like an actual dog whistle; a whistle that's higher pitch than humans can hear but dogs can.