Imputation is a statistical method that expands the information obtained from the chip. The Starter and Advanced chip reads around 750,000 markers; by comparing them with sequenced reference genomes from thousands of people, other variants commonly inherited alongside those markers are estimated, bringing the total to tens of millions.
We only use imputed variants that pass our quality checks. Imputation is highly accurate for common variants and less accurate for rare ones, so it does not replace sequencing: it provides much more information at an affordable price. If you want to read virtually all your DNA, Ultra sequences the whole genome.