At Amazon, they have a selection of flash meters, mostly the Sekonic series. They seem to be the standard. The one I will review here is the L-308S.
When I started photography with studio lights, I wanted to shoot in manual mode, for control of every setting. I tried to do all settings by taking snapshots, and view that on the camera's small screen and then adjust things. I remember trying to shoot a low key portrait and found some combination of shutter speed, aperture and ISO. I managed to take a series with a very dark mood. I thought it was great, I really liked the photos.
When I got some prints back from the local photo shop, they were very dark, too dark. This didn't work. So I ordered a flash meter, a Sekonic and I started using it. The (amount of) light on my photos is now right. Maybe there would have been some way else to achieve that, but this works for me. It is now simply - turn on the meter, trigger the flashes, set the values on the camera, take the photo, and finished. At least until something is changed in the setup of the light. Then meter again, and the new settings will be right again.
I can use the meter by setting the ISO, let's say at 200. Then I set the shutter speed at let's say 1/100. Then do the metering and I get the aperture setting right away.
But as I often want to have my aperture set at a specific value, let's say f:5.6 I then just take a metering at f.x. 1/100 and then click up or down, till the value for the aperture is f:5.6 on the meter reading, and then the variable - now the shutter, to what the meter shows, like f.x. 1/80.
This works if I have a specific value I want for the aperture, if i want a shallow, or a deeper field of focus. Photographing in a studio makes the aperture the most important for me. Shutter speed is of lower priority as nothing moves.
I then later - recently got me a new trigger set, where I use the trigger unit to fire the flashes for metering the light. I bought the same as what came with my studio strobe kit. Then I do not have to use the trigger on the camera, what would not work because I couldn't be holding both the meter and trigger the camera. Now I can leave one trigger on the camera hot shoe, and one in my hand to use with measuring the light. Earlier, another way to trigger the flashes would be to take a shot from the camera, by using a remote control to fire the camera and by that trigger the flashes.
That would also mean to "waste" a shot.
The flash meter here can also do metering in ambient light. Ambient light is the available light, like the sun, bulbs in the room; any light really that is not a flash. You just set the mode on the flash meter to continue, set your ISO and your chosen shutter value. Then press the button, and you will get the aperture setting that will suit that shutter.
This metering is what you should choose if you use continuous light, light that is always turned on, like if you do videos or take photos outside, without any flashes.
Most cameras have this kind of light meter built in. Meters that work with the ambient light, but not for flashes.
Here you will need a flash meter.
There is some argument about which way to turn the sphere on the meter. Some say in front of your subject, turned towards the camera. And some say towards the light source.
My personal religion is to point it towards the camera, as that is the point from where the camera sees it.