They prove that the idea of even a single strong impression may be so powerfully associated with that of a certain time, as to originate a belief of which the contrary is inconceivable, and which may therefore be properly said to be necessary.

A single weak, or moderately strong, impression may not be represented by any memory.

But this defect of weak experiences may be compensated by their repetition; and what Hume means by "custom" or "habit" is simply the repetition of experiences.
"wherever the repetition of any particular act or operation produces a propensity to renew the same act or operation, without being impelled by any reasoning or process of the understanding, we always say that this propensity is the effect of _Custom_.

By employing that word, we pretend not to have given the ultimate reason of such a propensity.

We only point out a principle of human nature which is universally acknowledged, and which is well known by its effects."-- (IV.p.