Here is Stallings' diagram of fast-frequency-hop spread spectrum.
Here is Stallings' CDMA example table. CodeA and
CodeB are orthogonal, and CodeA and CodeC are orthogonal, but CodeB
and CodeC are only "near orthogonal" (that is, their "dot product"
is 2 (which is small) rather than 0.
The dot product is formed by multiplying corresponding elements in
the codes (first, second, ..., sixth), and then adding these.
For example, the dot product of CodeA and CodeB is
1*1 + (-1)*1 + (-1)*(-1) + 1*(-1) + (-1)*1 + 1*1 = 0.
Recall that we multiply each user's code by 1 for a 1-bit in that user's message and -1 for a 0-bit, and then simply add the codes of each user (this simulates transmitting them simultaneously). If the codes were orthogonal, then taking the dot product of this sum and codeA gives either 6 or -6 depending on whether A sent a 1 or 0 bit, etc. If the codes are only "close" to orthogonal, the result may be "close to" 6 or "close to" -6; typically, though, all we need to know is whether it is >0 or <0.