ISO is a setting on your camera that lets you set how sensitive the sensor is. In brightly lit scene's you set it to lower number and as it gets darker you may need to set to a higher number in order to let enough light pass through the lens and shutter to achieve proper exposure. It is best practice use the lowest ISO you can in order to achieve proper exposure because the higher the ISO setting the more digital noise is recorded to the photo/file.
F stop/aperture and shutter speed control the amount of light let into the camera and onto the sensor or film and ISO is how sensitive the camera's sensor is to light.
Aperture is designated in stops or what is called Fstops, to change the size of the aperture you change to a different Fstop. The confusing part of this for beginners is that smaller numbers represent larger apertures and larger numbers represent smaller apertures. Meaning F2.8 has a larger size opening then F5.6. By changing the size of the aperture you are changing the amount of light that is allowed to pass thru the LENS. A larger hole allows more light to pass thru then a smaller hole.
Shutter speed is the amount of time that the shutter is open. This is a set of numbers that is based on time, 2 seconds is twice as long as 1 second. All time less then 1 second is represented in fraction I.E: 1/2 a second or 1/500th of a second etc. When you set your camera for 1/500th of a second the shutter opens to let light pass thru for 1/500th of a second.
So now you know that you can alter the amount of light by setting the Aperture ( size of the opening inside the lens ) and by setting the shutter speed ( the amount of time the shutter stays open )
Your camera has a light meter built in to it so it can measure the amount of light in a scene and then calculate how much light to let into the camera and on to the sensor based on the Iso setting you have chosen. When your camera is is on Automatic mode the camera takes the information from the light meter and it automatically sets the the Fstop and the shutter speed to give you the proper exposure for the amount of light in the scene you are shooting.
When you set you camera to manual mode the light meter is still taking a measurement of light in the scene but it is NOT setting the Fstop and the shutter speed automatically. YOU MUST SET THEM YOURSELF. You will look at the information that the light meter is telling you and MANUALLY set the Fstop and shutter speed to match that information.
One of the reasons to use manual mode is that your brain is more powerful and may contain more information than the cameras software does so you may want to set the Fstop to different value then what the light meter is telling you because your brain knows that there is information that the camera does not know OR you want to achieve a different exposure for artistic reasons.