This is the minimum number of shares that a market maker must make a firm price in. You can buy or sell fewer than this number of shares, in which case you will definitely deal at the price quoted. If you wish to deal in more than the NMS, you may have to except less than the market price if you are selling, and pay more than the market price if you are buying.
NMS is particularly important for smaller company shares and warrants which tend to be less liquid.
For a large blue-chip stock such as ICI, the NMS is 75,000 shares, which equates to 300,000 if the share price was 4, but for a smaller company with a total market capitalisation of a few million pounds, the NMS could be 1,000 shares which would equate to just 100 if the share price was 10p.