What does "closing date" for an offering mean?

The closing date of the transaction is on or about December 9th, 2020. On or after that day, it will not be possible to purchase any more shares of this registered direct offering of American Depository Receipts of this NNDM stock.

Does this mean the company is expected to be able to raise the target amount by Dec 9?

That depends on what type of registered direct offering this is. A registered direct offering may be structured as

1. “any-or-all” where the transaction closes regardless of how many shares are sold, or how the amount of proceeds raised;
2. as a “minimum-maximum” transaction, where a certain minimum threshold dollar amount must be raised in order for the transaction to close and a maximum offering size also is indicated;
3. as an “all-or-none” transaction, where all of the securities offered must be sold in order for the transaction to close.

The Form F-3 and the press release both say that the company has entered into definitive agreements with investors for the sale of the ADRs at a specific price, and that there is a single placement agent. The closing date is when sales stop, although I don't know which of the three sorts of registered direct offerings this is.

(The Form F-3 is dated December 4, 2020. I don't think the form being valid for three years has anything to do with this offering's closing date. If you look on page 19 of the SEC registration statement it looks like the company is saying that selling registered direct shares is one of many ways it can do stock offerings, so maybe that's why it is valid for so long, i.e. to cover future, separate offerings of stock.)