UVB helps reptiles synthesize vitamin D3 so that their bodies can use it. The D3 then helps to absorb calcium to their bones, helping to prevent many health issues such as metabolic bone disease. MBD can cause bones to grow deformed and weaker, which can lead to severe pain and death. It's also important to mention that, although D3 can be obtained through supplementation, such as calcium, that is not a replacement for a UVB bulb, and your reptile could still develop MBD if this were to happen.
There are different types of UVB bulbs you can use such as a coil bulb that screws into a light fixture just like a regular light bulb would, and linear UVB bulbs. I personally prefer linear bulbs similar to Zilla's Slimline UVB light fixtures, since those can cover more of your reptiles enclosure. This gives them the opportunity to absorb UVB even while not basking. However if you use a coil bulb, then your reptile can only adsorb UVB in one specific spot of the enclosure.