Help understanding what Gottfried Wilhelm Leibniz refers to as a monad?

The ground of the confusion lies in a misunderstood monadology, which does not belong to the explanation of natural phenomena, but is a platonic conception of the world, carried out by Leibnitz. This is correct in itself, in so far as it [the world] is regarded, not as object of sense, but as thing in itself; but is nevertheless a mere object of the understanding, though it lies at the foundation of the phenomena of sense. <METAPHYSICAL FOUNDATIONS OF NATURAL SCIENCE>

In an object of the pure understanding that only is inward - which has no relation whatsoever (so far as its existence is concerned) to anything different from itself. It is quite otherwise with a substantia phaenomenon in space; its inner determinations are nothing but relations, and it itself is entirely made up of mere relations. We are acquainted with substance in space only through forces which are active in this and that space, either bringing other objects to it (attraction), or preventing them penetrating into it (repulsion and impenetrability). We are not acquainted with any other properties constituting the concept of the substance which appears in space and which we call matter. As object of pure understanding, on the other hand, every substance must have inner determinations and powers which pertain to its inner reality. But what inner accidents can I entertain in thought, save only those which my inner sense presents to me? They must be something which is either itself a thinking or analogous to thinking. For this reason Leibniz, regarding substances as noumena, took away from them, by the manner in which he conceived them, whatever might signify outer relation, including also, therefore, composition, and so made them all, even the constituents of matter, simple subjects with powers of presentation -- in a word, MONADS. <CPR, Norman Kemp Smith>

The Leibnizian monadology has really no other foundation than that Leibniz represented the difference of the internal and the external in relation to the understanding only. Substances must have something internal, which is free from all external relations, and therefore from composition also. The simple, therefore, or uncompounded, is the foundation of the internal of things by themselves. This internal in the state of substances cannot consist in space, form, contact, or motion (all these determinations being external relations), and we cannot therefore ascribe to substances any other internal state but that which belongs to our own internal sense, namely, the state of presentations. This is the history of the monads, which were to form the elements of the whole universe, and the energy of which consists in presentations only, so that properly they can be active within themselves only.

For this reason, his principle of a possible community of substances could only be a pre-established harmony, and not a physical influence. For, as everything is actively occupied internally only, that is, with its own presentations, the state of presentations in one substance could not be in active connection with that of another; but it became necessary to admit a third cause, exercising its influence on all substances, and making their states to correspond with each other, not indeed by occasional assistance rendered in each particular case (systema assistentiae), but through the unity of the idea of a cause valid for all, and in which all together must receive their existence and permanence, and therefore also their reciprocal correspondence according to universal laws. <CPR, Muller>