When i see thing on the internet about aircrafts it often has a number at the end of the model number such as B737-300, what does the number after the dash mean?
In the case of the 737, the first series produced was the 737-100. Not many were made.
Then the 737-200 was built, which was very popular.
The third generation of the 737, known now as the "Classic", had three variants, the -300, the -400, and the -500. All are very similar, the significant difference being the size. The -300 has bigger (more powerful, as well as physically larger) engines than the older -200 did, and a longer cabin. With its success, Boeing decided to lengthen the cabin further, resulting in the -400. They also made a variant that was essentially the length of the shorter -200, and since it came along third it got the next designation, the -500.
The next generation of 737's, known as the "Next Generation", runs from the -600 through the -900, and this time Boeing decided to make the numbers correspond with the fuselage length, so the -600 is about as long as the -200 and -500 (roughly), the -700 is comparable to the -300, the -800 is longer as the -400 was, and the -900 is longer than anything before.
Now they're out of numbers, and so the NEXT generation of 737's, known as the Max, gets single digit numbers... the Max -7 (i.e. 737-7) corresponds roughly to the length of the -700, the Max -8 to the -800, the Max -9 to the -900, and so on.
The point of this long discussion is simply that the numbers following the dash represent different variants of the aircraft. In some cases, it may be a new generation design; in other cases it may simply be a different fuselage length.
As the suggested-as-duplicate question discusses, HOW those numbers are chosen is sometimes driven more by marketing than it is by mathematical progression. Often, though, the higher number indicates a later version, and often-but-not-always, a newer generation. But you really have to know the particular aircraft line to know which it is.
The number after the dash is basically just the variation of that aircraft type. For example we have the Boeing 777-300, however there is a newer model coming out called the Boeing 777-900.
It just helps differentiate the different models of that aircraft types.