Multivitamins are a form of dietary supplement containing (obviously…) vitamins. In general, they have been shown to be of use if you actually have a diagnosed deficiency in a particular vitamin, and folate and iodine specifically are recommended during and shortly before pregnancy. Vegans may also have to take a vitamin B12 supplement, as it’s one specific one that hardly occurs in any non-animal foods (only algae I think). However, there is no real evidence that taking multivitamins is of any benefit outside these scenarios.
If at all possible, getting your vitamins from eating food is greatly preferable to taking a multivitamin, as it is very difficult to overdose on a vitamin purely from the quantities you’d get in food (not impossible though: be careful if you eat liver), and your body can make better use of vitamins in those forms than in the artificial, shelf-stable forms that supplements tend to make use of.
Some vitamins do cause damage if taken in large quantities, like vitamins A and B3 (niacin). This makes it a bad idea, generally, to take the kind of “all-in-one”, broad-spectrum multivitamin loaded up with like 10,000% the RDA of every vitamin. These kinds of products can actually do more harm than good, causing stress for the liver.
More moderate multivitamins than those are unlikely to do any harm; it’s just they’re not overly likely to do substantial good, either.
The poor regulation of multivitamins is a matter for controversy in a number of places. In the US and EU, they seem to be regulated as food, not medicine. They’re required to be safe, but are not necessarily required to be “effective”. (Although many countries require companies not to claim effectiveness for things they can’t back up with evidence.) In Australia they do come under the purview of the TGA, and manufacturers are expected to be able to supply proof of any claims they make if requested, but they’re not required to supply proof of their claims just as a standard part of the approvals process, so in practice it takes a long time for any incorrect claims to be called out and then they can get away with doing a minor rebrand to start the process over again.