"&" is a binary AND operator: it combines bits in the operand, so if the bit is one in both operands, than the result is the same bit one - other wise it is zero.
So, for each bit:
Op1 & Op2 Result
0 0 0
0 1 0
1 0 0
1 1 1
It's kind hard to see from your example what is going on, because all your numbers are in decimal - which doesn't relate "nicely" to the actual bits. But...if you translate them to binary values first it's more obvious:
Your command Op1 Op2 Result
select 1&1 01 01 1
select 1&2 01 10 0
select 1&3 01 11 1
select 1&4 001 100 0
select 123&1 1111011 0000001 1
select 123&7 1111011 0000111 11
etc.