The miswak is a twig made from the Salvadora persica plant. When one end is cut into a bristly brush, it can be used to clean teeth in a similar way to a toothbrush.

There are multiple hadith about Muhammad recommending that his followers use the miswak for cleaning purposes. You can find them here. Just so you know, not all hadiths are regarded as genuine by either historians or Islamic scholars. They're like second hand reports of what Muhammad did and they vary in theological and historical authenticity.

An example:

Abu Hurairah narrated that:
Allah's Messenger said: "If it were not that it would be difficult on my nation, then I would have ordered them to use the Siwak for each prayer."

Though I don't know how widespread the use of this was, it's at least one way people cleaned their teeth prior to toothbrushes. Interestingly, some very devout Muslims in the modern day have taken up the habit of brushing their teeth with the miswak in order to emulate Muhammad.

Whilst I was looking for an exact quote from Gerald of Wales to give an, at least partial, answer to this question I came across this earlier thread in which /u/RevengeofTim answers the question for parts of medieval Europe.