Sorry. Think of it as the diameter of the beam of light leaving the binos and going into your eye. Your eye only gets the light that comes through the eye's pupil, though, so if the incoming light beam is larger that extra light is wasted. Hence my point about night glasses not being too useful for us old folks whose pupils don't dilate so much any more.
Optical engineer here. Oldshooter is giving excellent advice. I suspect he learned about rifle scopes at one point. That's one of the first things we learn to design.
I'm going to offer a slightly different explanation of exit pupil. Oldshooter is 100% correct but it may still be hard to follow what he is saying.
A "pupil" in an optical system is an interface point between subsystems. Think of it like the connector behind your TV where you plug the HDMI cable in. Electronics are easy because we use cables. In optical systems we are passing light rays so aligning one part to the next is important.
Binoculars collect light at the entrance pupil (the big objective lens), relay it and magnify it, then deliver it to your eye(s) at the exit pupil of the binocular which you have to align with the entrance pupil of your eye manually when you pull them to your face.
At the exit pupil, any ray of light that came directly into the objective "on axis," meaning directly from the center of the deer, sparrow, or quarterback you were looking at will still be at the center. But anything "off axis" will be spread out in the exit pupil/focal plane. So, as Oldshooter says, if your pupils only dilate to 3 mm and you have a 7 mm exit pupil, you are missing a lot! It can also be hard to properly align to your eyes so you may not get the most out of a good pair. (This is technically not quite accurate but I am trying not to write a book.)
Your eyes also have entrance pupils. To use a binocular you need to be able to position each side of the binocular in the right place so its exit pupil aligns with each eye's entrance pupil, and they will be different for pretty much anyone, so having the right differential adjustments and learning to use them for YOU is important. Most but not all binoculars will have a differeential focus on one eyepiece to allow for this.
Other things to consider, the first number is objective lens diameter and the second is manification. Bigger objective means more light gathered and will be better under darker conditions but also bulkier and heavier. More magnification brings the deer, sparrow, or quarterback closer but also makes it harder to keep steady.
I own 3 pairs, a Bushnell 7x50 I got from my dad so vintage 60s I think, a Bushnell 7x50 I bought for myself in the early 80s, and a Bushness compact 7x35 I bought in the late 80s. Both my purchased pairs are good quality and cost about $100. My dad's pair are by far the highest quality but the lenses have begun to fog. Of my pairs I rarely use the big pair and stick with the compact.
In terms of advice, I consider Tasco a "toy" brand that is poor quality. I would be surprised if you could get $5-$10 for a used pair. You may do better donating them to a local school or nonprofit and taking a tax deduction. After that, I would keep the ones that YOU like best.
My partner recently bought a cheap pair of binoculars for about $20 withought even talking to me. Not that she needs to ask me to spend money but jeez, I'm an optical engineer! Hers are compact Bushnell 7x35 I think and they are quite good.
We’ve each had the 8.5x42 bins which has been an absolutely outstanding birdwatching optics we’ve enjoyed for decades. More recently we’ve each bought a lighter, more compact 8x32 bins which we prefer for traveling.
