shift register

its part of code for shift register

you should look at the FULL code, then it can be explained too

bit_out <=bit_out(30 downto 0) & '1';

This is a simple signal assignemt. It takes the lower bits 30-0 of 
bit_out and concatenates it with the constant '1' at the lowest bit 
position. This is assigned to bit_out again. This is a logic shift left 
operation, which feeds a logic one into the LSB (least significant bit).
Another cumbersome way to describe it would be like this.

bit_out(31) <= bit_out(30);
bit_out(30) <= bit_out(29);
bit_out(29) <= bit_out(28);

...

bit_out(1) <= bit_out(0);
bit_out(0) <= '1';

The & operator is the concatenation operator. It connects bits and/or 
vectors to a new vector.

It is not the AND operator as in C.