The term Aliasing refers to the jagged edges sometimes seen in graphic images, especially around curves. These are often called jaggies.
On the example pictured right, you can see how the letters of the first word have ugly jagged curves. The only letter not affected is the "T" because it has no curves.
The second version of the word has been anti-aliased. This is a process of smoothing the jaggies by introducing new graduated colours to the edges of the image.
The following example is a close-up of part of the "x". You can see how the original image consists of two colours: blue and a white background. The anti-aliased version includes several additional shades of blue which help blend the edges into the white background.
Anti-aliasing is supported by all good graphics applications. Most have more than one anti-aliasing option and/or various parameters to adjust the amount and quality. You may need to experiment to get the best results for each image.