Every DSLR camera (like yours) has a few standard modes... but before you can understand the modes, you need a little background on how "exposure" works on a camera.
The camera needs light to produce an image on the sensor inside the camera. It controls the flow of light into the camera much in the way a faucet controls the flow of water from the tap.
In the case of the tap... you could... barely open the tap and let the water trickle out for a long time to "fill your glass"... or you could crank that valve wide-open and let the water come gushing out and it would fill your glass a lot faster. It's also possible to over-fill the glass and make a mess.
Same holds true of photography...
There is a setting that controls how long the shutter remains open when you take a picture.
There is another setting that controls the size of the aperture opening in the lens (but the lens obviously has limits to just how small or how large this can be and it varies by lens -- but within that range, you can control it.)
Lastly... there is also a setting that controls how much light the camera should receive in order to have a satisfactory exposure. You can think of this as a setting that controls the sensitivity of the image sensor. In our "water glass" example where we have to fill the glass to get a good exposure... this would be like changing the size of the water glass. BTW, to say that it controls the "sensitivity" is the simple way of explaining it... in reality it doesn't alter sensitivity, it works more like a kind of "amplifier" (but to keep this reply short, we won't go into the nuances of how it really works and will just say it deals with "sensitivity").
So that's basically it... (1) aperture opening size, (2) time that the shutter is open, and (3) sensitivity of the sensor.
When the camera is in "M" mode (Manual mode), you control all three settings manually.
When the camera is in "P" mode (Program mode), the camera sets all settings automatically but it does give you some latitude to over-ride things (features such as "program shift" and "exposure compensation"). If you have no idea what settings to use and just want to pick something "safe"... this is the setting to use because the camera will try to find a "safe" (likely to be good) exposure for most shots (whenever possible... in some extreme shooting situations you can back the camera into a corner and deprive it of being able to use a "safe" exposure. )
The downside to using "P" all the time is that you can usually get an even better exposure than the "safe" exposure, if you understand how to tweak some settings.
The other two modes... Tv and Av are modes where you pick one setting and the camera picks the other.
Tv stands for "Time value" and this means you want to set the shutter speed, but have the computer pick whatever aperture is needed to create a good exposure. Be careful here because as previously mentioned, there are limits to just how large the aperture opening in the lens can be -- so it's possible to set a time value for which the corresponding aperture needed cannot be achieved by your lens.
Av stands for "Aperture value" and this means you want to set the aperture opening size, and have the computer pick whatever shutter speed is needed to create a good exposure. This is usually safer because the camera can handle pretty fast shutter speeds (I think your camera model can handle speeds to 1/4000th second) and there's no practical limit on the maximum exposure duration (the camera's shutter speed goes to 30 seconds but it's possible to take long exposures with a the "bulb" setting (which relies on external input to control the shutter duration.)
Now you know what those settings "do"... but you don't really have any examples of "why" you'd pick one mode over another. That's more than I can explain in a post.
I suggest you pick up a copy of "Understanding Exposure" by Bryan Peterson. The book is designed for beginners. He doesn't use terminology that requires any previous photography training to understand. It will walk you through not only how those controls work... but why you'd use them (with practical examples.)
You may find this video to be helpful:
ive been doing photography for number of year, but this is the first camera where you need to know the exposure haha
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