Block alighment, in the context of sofware engineering/computer science, is putting data in memory at addresses that are a multiple of 4 (or whatever the word size is, x86’s word size is 4 bytes).

For example, if you have the following data

struct {
    char a; // 1 byte
    short b; // 2 bytes
    int c; // 4 bytes

This struct should be laid in memory as follows:

  address   content

     0 ───── char a
     4 ───── short b
     5       short b
     8 ───── int c
     9       int c
     10      int c
     11      int c

Notice, that regardless of the size of the data (1 byte, 2 byte, 4 bytes, etc), its address in memory always starts at a multiple of 4.

CPUs can fetch content at address of 4 multiples significantly faster than at other addresses.

For this reason, when you wanna save data to disk, or send it over the network, you “compress” it (make it non-block aligned) to save space, but when you load it back into memory again, you should block align it for speed.