Read This Disclaimer Introduction Bilinear interpolation is a process that enhances textures that would normally be larger on-screen than in texture memory, i.e. a single texel is used for more than one screen pixel. Regular texture mapping would appear to be 'blocky' in this case.
Bilinear interpolation reduces the blockyness by interpolating between texels.
During texel fetching, usually only the integer part is used to index the texels. This process enhances enlarged texture maps, but also rotated texture maps, because for these textures effectively the pixels on screen are anti-aliased. Obviously, this process requires substantial calculations per pixel. First, four weight factors must be determined. Then the 4 texels must be fetched and multiplied by these factors.