Ben Jonson's contemporaries seem not to have understood it either. And here is Ben Jonson's own explanation:
I remember, the Players have often mentioned it as an honour to Shakespeare, that in his writing, (whatsoever he penn'd) hee never blotted out line. My answer hath beene, would he had blotted a thousand. Which they thought a malevolent speech. I had not told posterity this, but for their ignorance, who choose that circumstance to commend their friend by, wherein he most faulted. And to justifie mine owne candor, (for I lov'd the man, and doe honour his memory (on this side Idolatry) as much as any.) Hee was (indeed) honest, and of an open, and free nature: had an excellent Phantsie; brave notions, and gentle expressions: wherein hee flow'd with that facility, that sometime it was necessary he should be stop'd: Sufflaminandus erat; as Augustus said of Haterius. His wit was in his owne power; would the rule of it had beene so too. Many times hee fell into those things, could not escape laughter: As when hee said in the person of Cæsar, one speaking to him; Cæsar thou dost me wrong. Hee replyed: Cæsar did never wrong, but with just cause: and such like; which were ridiculous. But hee redeemed his vices, with his vertues. There was ever more in him to be praysed, then to be pardoned.
Ben Jonson's point being, as this blog explains, that if he wrote as well as he did without revising his work, just think how much better he would have written if he had revised it.