Logos can be vaguely translated as the reason for things, the word 'logic' comes from the root word 'logos' or 'the reason of things'. In philosophy it is defined as the cosmic reason or the cosmic rationality implying an order and intelligibility to all things and events in terms of their value and perfection.
Logos already implies some kind of meaning in the form of rational thought. For the logos, reason and thought already have some kind of privilege significance in describing being and moreover the relations of being: logos as the reason, the rationale of the world, the nature of the world.
The logos in addition to implying this kind of rationale this kind of rational mediation of the world principally through words and through language is also going to be identified in another way: logos by default and upon its inception implies an exclusion, logos is by nature exclusionary and the way in which logos is exclusionary is that logos in addition to identifying the reason, the rationale the meaning behind things also says that this reason, this rationale is its form, is its identity, beyond which there is nothing and nothingness.
For Heraclitus and at the outset logos basically meant the consistency of inconsistency and the 'identity of difference' by consequence. Heraclitus teaches us that the world is inherently somehow inconsistent every time we try to ground the world or freeze the world into something some kind of form, let's say phenomenal form. The world will necessarily be inconsistent with that because it's always changing and it's always in motion: 'no man steps in the same river twice'.
It is the permanence of what is impermanent that allows Heraclitus to derive this concept of logos. Logos therefore never entailed from the very beginning any kind of substantive being, logos from the very beginning, as the beginning of it all, already entailed some kind of consistency of inconsistency alone. The content of logos can only be defined by something which is 'not what it is'.
Heraclitus is also known for arriving at the insight that 'all is one' because for Heraclitus he is dealing with a world that has already been lacerated into distinctions and differences. So, it is the all of difference, all of these different things that is in the end 'one', that 'one' for Heraclitus it is the one of difference itself. [1]