A "digestive biscuit" is sweeter than a (US) cracker but not as sweet as most cookies. It's very plain & simple (like a graham cracker) and is (I presume) considered: A) easily digestible or B) good for the digestion or C) both the above
My (English) husband adds, "It's that little bit softer than a 'rich tea biscuit'", which I'm sure makes it all *much* clearer, yes?
The American-British/British-American Dictionary says: "digestive biscuits n : dangerously addictive drug with no known remedy. Actually, it's just a semi-sweet wholemeal biscuit, and you can sometimes get them at Safeway, but they probably epitomize what any British person thinks of as a biscuit, and for those addicts, a cup of tea and biscuits ends up being as many cups of tea required to consume the whole packet. (So there!)"
(If you look up "graham crackers", it says "these are like a semi-sweet biscuit, similar in texture to a light digestive biscuit, and eaten as is, and also often crushed to make a pie crust."
If you want to see a picture of a digestive biscuit (sort of -- it's on the packaging), click here.