A lens' maximum aperture size is frequently described as a ratio of the lens' focal length (f). Thus, the
EF 50mm f/1.8 lens has a focal length of 50mm, and its maximum aperture is 50mm / 1.8 = 28mm. A bigger aperture lets in more light, allowing a shorter exposure. A smaller aperture lets in less light. Each f-stop cuts the amount of light received in half (f/1.4, f/2, f/2.8, f4, f/5.6, f/8, f/11, f/16, f/22, etc.), requiring twice the exposure length. A wide aperture also creates a shallow depth of field, affecting the range of subjects that can be in focus at the same time.
Your EOS Rebel XSi probably came with the
EF-S 18-55mm f/3.5-5.6 IS zoom lens. At 50mm, its maximum aperture of f/5.6 lets in 1/10th as much light as the f/1.8 lens can.
A well-regarded lens for shooting wild birds is the Canon
EF 100-400mm f/4.5-5.6L IS USM. This high quality telephoto zoom lens has a range of focal lengths between 100mm and 400mm, a maximum aperture of f/4.5 at 100mm, a maximum aperture of f/5.6 at 400mm, and it includes optical image stabilization and an ultrasonic focusing motor. At over $1400, it is too expensive for a starting lens.
A relatively inexpensive telephoto zoom lens is the Canon
EF-S 55-250mm f/4-5.6 IS. This isn't a high quality lens, but at around $265, it's more suited to a beginner's budget.
Many folks recommend the high quality Canon
EF 70-200 f/4L USM for $600 as a great telephoto zoom lens.
I have the Canon
EF 70-300mm f/4-5.6 IS USM. Quality-wise, it's between the EF-S 55-250 and the "L" lens, but it does include optical image stabilization, for about $550.