You need to take "good funds" to the closing.
A cashiers check is the most common form of good funds, but you can also make arrangements to wire your funds to the title agent. If you plan to wire, try to have the funds there a day ahead of time to avoid complications.
There are options for good funds. You can use a money order, official bank check, or teller check. Title agents who wish to avoid risk will not take cash or certifed checks.
If you are selling a home and buying a home at the same time, have a chat with the title agent handling the sale. Make certain they will provide good funds. They are required to do so, however, some will argue the point. Expect to pay a small fee for the wire or cashiers check.
Oh, and BTW, make the cashiers check payable to the title agent or to yourself. Do NOT make it payable to the Realtor or the mortgage lender. No matter how many times we remind folks, we still have to send some back to the bank to have their check reissued.