Exposure is term that indicates amount of light sensor has recieved in correlation with how wide was aperture open and through what time period. So simply said exposure is correlation between aperture (F), exposure time and ISO value. I will try to scenically demonstrate on bib-cock example. To fill certain canister to top it is all the same whether you are going to open the bib-cock all the way or just a little bit (small aperture) and let the water flow through longer time period (exposure time) or will you open the bib-cock all the way (large aperture) and let the water flow through shorter time period. In both cases amount of water in the canister is equal. In this picturesque example water = light, bib-cock opening = aperture, and water flow time = exposure time. Conclusion is that we will get the same amount of light with larger aperture and shorter time, or with smaller aperture and adequate longer time. Therefore, regarding light, it is completely the same will you have F4 and 1/100 or F2.8 and 1/200. In addition to that, let's say that correctly exposed photography equals full canister of water. Overexposed photography equals overfilled canister, because there is too much light respectively water, whether the reason to that is to large aperture or too long exposure time. I explain all of this just so you could understand the relationship of these two parameters. According to that, underexposed photography equals unfilled canister because there is not enough water in it as there is not enough light on photography. Reason to that lies evenly in aperture/bib-cock and exposure time. Either the bib-cock/aperture should be opened more or the pouring/exposure time should be prolonged, or both. In this section we won't occupy with sensor sensitivity (ISO), which is tightly connected with exposure, because it would puzzle the understanding of this term even more.

Graphical explanation of Exposure