What do the number at end of model numbers 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.