Yes, EI is Exposure Index. It's a description of how much you exposed the film (bigger numbers are less exposure), so it is a function of the meter reading, the aperture and the shutter speed.

Simplest way to achieve a specific EI is to set your meter's ISO setting to that EI, take a light reading and put those settings (aperture and shutter) onto the camera.

ISO is a definition of how much exposure a film should get for normal exposure and development, EI is how much exposure you actually give it. Most people find that Fomapan is quite optimistic in its ISO rating, i.e. they get less shadow detail than they want, so they expose it at EI50 instead of EI100. 50 is twice as much exposure, i.e. one stop more, which gives more shadow detail and a denser negative.

Usually when adjusting EI, one would also adjust development. You don't just randomly pick an EI, you typically decide on how much contrast you need for a particular scene, which defines how much development you're going to give that sheet, which defines the appropriate EI. Have a read about Zone System and Beyond The Zone System (BTZS).