Could someone tell me what the define block in more blocks do?

So, when you have a custom block, for this, lets call it advance, Scratch doesn't know what ‘advance’ means.

Thats where define comes in. We need to define ‘advance’ for the program, therefor telling it what it means, and what to do.
In this case, Advance will mean to switch to the next costume. (You don't have to do this, its just to help, lol)

So, when you place/connect your own custom block to something…

It will do as told, because we defined it to do so.
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Understand? Hope this helped!

They might be a little hard at first, but it will make sense later!

This crops up regularly - eg https://scratch.mit.edu/discuss/topic/111228/

Simple example - you could create a custom block to draw a circle of a certain size centred on an x and y position. If you used a broadcast for this you'd need to always use the same variables or the same values but a custom block can be called with the values you want or with different variables (so one time the size could be variable size1 and the next it could be size2).

You can also run them without screen refresh which makes things run much faster.

Oh, I get it now! Thanks a lot for your help!