While the existing answers cover the most common uses of such codes (ICAO and IATA codes,) in order to further complicate things, there is actually a third possible meaning, as well. Airlines often assign a letter designator in order to specify a particular cabin configuration. For example, Delta has no less than 9 unique codes for its 757-200 cabin layouts, even though they're all exactly the same airframe. If you're looking at a website like SeatGuru (or the website of the airline itself,) these are the codes you'll see in reference to exactly which cabin configuration will be on the particular flight you're booking.
A given seat number may refer to a completely different seat or even not exist at all from one cabin configuration to another, even on the same type of airframe on the same airline, so these codes are used to disambiguate the configuration. Unfortunately, this also has the result of making the code itself even more ambiguous in the absence of context indicating which type of code it is.
While this type of code doesn't mean much to the pilot, it can mean a lot to the passengers. To go back to the Delta 757-200 example, on the 75A configuration, seat 2A is a normal First Class seat with 40" seat pitch. On the 75S configuration, on the other hand, it's a 76-inch-long flat-bed seat. Slight difference there. While the differences in economy usually aren't quite as dramatic, they can still be rather large. For instance, on the 75S configuration, seat 19F has a lot of extra leg room due to a missing seat in front because of an exit door, whereas it's just a normal economy seat on the 75A configuration.
As far as I know, this code is distinct from the customer codes discussed in this question, which would instead take the form of 757-232 for a 757-200 delivered to Delta Airlines. The code discussed in this answer denotes the current cabin configuration of the aircraft and takes the same form as the ICAO/IATA codes, whereas the code discussed in the other question denotes the customer who originally ordered the aircraft from the manufacturer. So, while all 757-200s initially delivered to Delta Airlines would be 757-232s, they can (and do) have many different cabin configurations, each with their own code of the form 75A, 75S, 75Z, etc.