Harker doesn't get his Shakespeare quite right. It should be "My tables, – meet it is I set it down". Hamlet is reacting to some unpleasant revelations about his mother and uncle, delivered by the ghost of his father; his immediate urge is to write down what he has heard in his notebook – or "tables".
Tables were a portable writing solution in an age when paper was very expensive and quill pens were impractical for use on the move. They represented a remarkable, and largely forgotten, technology, with specially treated pages that could be written on with a metal stylus and then wiped clean with a sponge. The pages, sometimes called "asses' skin", seemed to have been wax-coated vellum or card.
A rare specimen can be seen here
Understanding this erasable technology makes much more sense of the speech, clarifying the metaphor at the beginning and the physical item Hamlet seeks towards the end:
Yea, from the table of my memory
I'll wipe away all trivial fond records,
All saws of books, all forms, all pressures past,
That youth and observation copied there;
And thy commandment all alone shall live
Within the book and volume of my brain,
Unmix'd with baser matter: yes, by heaven!
O most pernicious woman!
O villain, villain, smiling, damned villain!
My tables,--meet it is I set it down,
That one may smile, and smile, and be a villain;
At least I'm sure it may be so in Denmark
Hamlet Act I, Scene 5