Aperture refers to the opening the light will pass through to get to the sensor or film and is used in conjunction with the shutter speed to determine the exposure of your photo...a "fast" shutter speed usually would require a larger opening to get enough light to the sensor and a "slow" shutter would naturally require a smaller opening...
When you hear the word "f stop" or see" f number" in relation to a lens , that is telling you the diameter that lens can open to, such as f1.8, f28, f4, f8, f11,f16,f22 and are really ratios so the smaller the number ( IE f2.8) the larger the diameter of the opening or the larger the aperture. If it is given as a ratio IE f3.5-5.6 that means at the widest angle( the shortest focal length such as 28mm) it has a top aperture( biggest opening) of f3.5 while at it's narrowest angle (the longest focal length such as 100mm) it has a top aperture of f5.6. Any lens would have a set amount of f stops, such as f2.8-f22, depending on the lens
Why should you care? First a "fast" lens, one with a large aperture or small f number such as 2.8 allows you to shoot in lower light situations since it allows a faster shutter speed due to the larger opening. I f you recall to keep a sharp photo you need to be able to set a shutter speed at least the approximate length of the lens, IE a 200mm lens a 1/200 shutter speed. At f8, you may not be able to get that fast a shutter speed due to the smaller amount of light going through the smaller aperture. That is one reason lenses with a larger aperture IE F2.8 are more useful, they can help you have a larger range of light conditions in which you can get a good photo.
Another benefit is, in general, apertures allow you to control the amount of the image that is in focus in the photo. If you want to take a portrait you want to have a smaller "depth of field" ( or "dof", the area of the photo that looks sharp front to back). Say you station the subject in front of a large bush to take their portrait... If you use a small f number ( IE f2.8, f4, so a large aperture) rather than a detailed, sharp set of leaves behind your subject you will get a nice bokeh, or blurred background which will make your subject stand out better and reduce any distraction behind him. Conversely, a larger f number is great for landscapes where you want clarity front to back.
More on depth of field some other time but in general, for something like a portrait or when you need a smaller dof...( as in second photo above,"Faded Rose " , f 3.5) you can decrease the dof by using a larger aperture and increasing magnification either by moving closer or using a longer focal length lens( IE 100mm). For something like a landscape(as in top photo, "Going Home", f11) you want a larger dof so use a smaller aperture( ie f11+), and a shorter focal length such as 28mm to give you more front to back sharpness.
The last camera setting you really need to know about to determine exposure for your photo is ISO...so next time.....
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