"Octet" comes from an group of eight bits, often also called (unsigned) "byte". An octet can have one of 2^8=256 positive values, 0 through 255. An IPv4 address is actually 32 bit, usually represented as four dotted octets for human readability.
Not all values can be used in the first octet of a host address, as several are reserved for special purposes. Most significantly,
* 0 is reserved for the "current network"
* 127 is reserved for the local host's loopback address (most often only 127.0.0.1 is used)
* 224-239 are reserved for multicast (as destination address only)
* 240-255 are reserved for "future use" (which isn't going to happen); 255.255.255.255 is used as the _limited broadcast_ destination address, ie. _all hosts in this network_
As a side note: the linked page talks all about _network classes_ which are long obsolete and may be interesting for historical reasons only. **Network classes were obsoleted decades ago** by CIDR (RFCs 1518 & 1519).