An idiolect is a person’s specific, unique way of speaking. Everyone has their very own idiolect that differs from the way other people talk.
A dialect is a version of a language spoken by a group of people. An idiolect is much smaller — it’s the way a particular person speaks, at a specific time, as distinct from others. This word is mainly used by linguists when discussing differences in speech from one person to another. Like your fingerprint, your idiolect is unique. It’s kind of like a micro-dialect.