It is where they put .99 on the end of products and/or in doing so, lower the price by 1%.
Retail prices are often expressed as "odd prices"
For example, £19.99 is perceived by customers as lower than it actually is. They think that they are closer to paying £9, instead of £10. It could make the sale because they may think £10 is too much but 1% lower is okay.
Disadvantages- it can be complicated when calculating costs because some customers check their receipts and its hard to add numbers which aren't whole. Also, it doesn't work on everyone.