Aperture is the diameter of the main mirror or lens, and it is the specification that decides what a telescope can show. Everything else on the box can be changed later with an accessory; the aperture is fixed on the day you buy it, which is why this index publishes it beside every price.
What aperture actually controls
Two things. First, light grasp, which goes with the area and therefore with the square of the diameter: a 130mm mirror gathers nearly three times as much light as a 76mm one, not twice. Second, resolution, the finest detail the instrument can separate, which goes with the diameter directly. Faint objects need the first; planetary detail and double stars need the second.
Why magnification is not the same thing
Magnification is set by the eyepiece and can be anything you like. What limits useful magnification is aperture and the steadiness of the air, and the working rule of thumb is about two times the aperture in millimetres on a good night: roughly 150x for a 76mm instrument, 260x for a 130mm. Past that the image gets bigger and worse, which is why the magnification figures printed on department store boxes are meaningless.
Large aperture and the practical ceiling
More aperture is always optically better and often practically worse. A 250mm dobsonian shows far more than a 150mm one and weighs enough that many owners stop carrying it outside. The instrument that gets used beats the instrument that is better, so the honest ceiling for most people is the largest thing they will move without thinking about it.
Questions people ask about telescope
What does telescope aperture mean?
The clear diameter of the main lens or mirror, usually in millimetres or inches. It sets both how much light the telescope collects and how fine a detail it can resolve.
Is a larger aperture always better?
Optically yes, practically only if you still use it. Weight, bulk and setup time all rise with aperture, and an instrument that stays indoors shows nothing.
What aperture do I need to see the planets?
Any of them will show Jupiter's moons and Saturn's rings. For belt detail on Jupiter and the Cassini division in Saturn's rings, 100mm and up on a steady night makes it comfortable.