During a plasmid miniprep, this ratio usually indicates the relative purity of the eluted DNA by comparing absorbance at 260nm, where nucleic acids have absorbance maxima, to 280nm where proteins have absorbance maxima. Inefficient neutralization or insufficient washing may allow excess protein to remain associated with the matrix and elute with the DNA. Please be sure to follow the protocol. Additionally, using the supplied elution buffer to elute the sample as well as blank the spectrophotometer will ensure there will not be a shift in the 260/280 due to sample acidity. If water is used to elute the DNA, use the same water as a blank on the spectophotometer.