Pink tax is an alleged surcharge for women's products above those used by men. It refers to the idea that certain products, such as lady's razors with pink handles, cost more than equivalent male products, such as men's razors with black or metallic handles.
In practice it is hard to compare products except in certain specific areas. Many products used by women do not have exact male equivalents.
Apparent differences may also be explained by factors such as men's willingness to argue over prices, or women's desire to purchase something "reassuringly expensive".
Factors affecting comparison
It's not simple to answer whether women's products really cost more. If it was just a case of seeing a pink thing and a blue thing that were functionally identical but with different prices, then it would be easy to judge the truth. In practice many things affect price and make judgment harder.
- Complexity. Many men will object that a man's clothes often cost more than a woman's. This applies in part because a trouser suit is far more complex to manufacture and tailor than a day dress; trousers cost more to make than leggings or skirts; etc. With formalwear, a woman's ballgown can be vastly more expensive than a man's dinner suit, again because of the additional fabric, stitching, and design work.
- Economies of scale. If one sex buys millions and the other buys thousands, then you would expect the better-selling to cost less, due to economies of scale, with fixed costs (design, tooling, advertising, etc) being spread over a larger number of products. You would expect this to apply regardless of the gender: if women use more cosmetics and skincare products, they should be cheaper than the male version; if men use more sports equipment, that should be cheaper than the female version.
- Size. Men are normally slightly taller and weigh a bit more. This may affect things that have to be scaled to fit body size; also if airplanes charged more for heavier people you would expect men to pay more.
- Males' clothes is more standardised. A man's shirt is pretty much the same, shirt by shirt, year by year. A woman's has much more variation.
- Single-sex only. Some products will seldom or almost never be bought or used by one or the other sex: tampons for instance. It is impossible to compare these. Bras likewise have a structure not found in any male clothing.
- Different ingredients. Some things like fragrance are very difficult to compare because they may include expensive ingredients that may or may not be clearly listed, but are formulated differently for the two sexes.
- Prices set by the market. This may or may not be convincing, but if women are willing to pay more, is it fair for them to be charged more? Traditionally, women have placed higher priorities on some areas. Men may be more confident at negotiating and willing to drive for a bargain. Either sex may assume something that is more expensive is better quality or trustworthy. On the other hand, apologists claim women may be charged both for areas they have considerable expertise and ability in, such as hair or beauty, and things they have traditionally little expertise in, such as car repairs.
On the other hand, even if items are incomparable, it is sometimes possible to consider factors like the profit margin on a product, or the value of raw materials and labor in comparison to the final product.
Cited examples include:
Dry-cleaning a man's shirt vs a woman's blouse: £162 v £310. This is explained as being due to the standardisation of male shirts compared to female. 
Perfume. Jean Paul Gaultier EdT for men is half the price of women; this may be explained by the more expensive ingredients, or by the fact that women are more likely to buy luxury or expensive products.
Car repairs cost more for women.
Haircuts. This may be explained by women's willingness to pay more, because women don't trust cheap haircuts, or because men's haircuts are simpler (though this is not always the case).
Deodorant even when from the same company.
The phenomenon leapt to public attention in 2015.
It may reflect an assumption that the male product is the normal, and having to accommodate women is an annoyance for which people will be charged extra.