A thought maker is a device that is used to alter the brainwaves of a person. It can either be used by a person on themselves, or it can be used by one person to manipulate another person.
In 2364, the Ferengi DaiMon Bok used a thought maker on Captain Picard as part of his plans to take revenge on Picard for the death of his son. Bok manipulated Picard into boarding the USS Stargazer, in which Picard defeated Bok's son nine years earlier, in order to have Picard attack his own ship, the USS Enterprise-D. Fortunately, Data was able to prevent Picard from attacking the Enterprise, and Riker was able to get through to Picard long enough to have the Captain destroy the thought maker. When the Ferengi found out what Bok was doing, Bok was relieved of his command. (TNG episode: "The Battle")
Four years later, Romulans used a thought maker to try to coerce Dr. Crusher, Ro Laren, and Deanna Troi. While under the influence, they attacked several members of the Enterprise crew while making their way to the battle bridge in an effort to take the Enterprise to a new destination. Riker was able to break the control by reaching out to Troi and calling her "Imzadi". (TNG - Intelligence Gathering comic: "Matters of the Mind")
Two years after that, Bok – who had bought his freedom some time ago – returned with a second plan to take revenge on the Enterprise. When Bok appeared to the Captain in private, Picard was concerned that Bok was again using thought makers. But a scan by La Forge revealed that thought makers were not being used. Instead, Bok was using a subspace transporter to actually board the Enterprise. (TNG episode: "Bloodlines")