What does it mean to have a subnet mask /32?

/32 addressing

Generally speaking, /32 means that the network has only a single IPv4 address and all traffic will go directly between the device with that IPv4 address and the default gateway. The device would not be able to communicate with other devices on the network.

There are a couple of possible reasons for this that I've seen. It could be:

• A webserver serving multiple sites with each site bound to a specific IPv4 address
• A loopback address used for testing.
• Isolating a machine from the network to allow only statically set routes to connect. (For decommissioning, for example.)

Network ID

The network ID portion of an IP address is determined by the subnet mask. For example:

• A /24 IPv4 network has a subnet mask of 1111.1111.1111.0000, meaning the first 3 octets are the network ID and the last octet is used for assigning host IDs (256 available IDs, though usually some are reserved).
• A /16 IPv4 network has a subnet mask of 1111.1111.0000.0000, meaning the first 2 octets are the network ID and the last octet is used for assigning host IDs (65536 available IDs, though usually some are reserved).

In the case of /32, this doesn't apply as the address is both a network ID and host ID. /31 addresses are also all host IDs with no reserved 0th address.

There's a bit of confusion here; that /32 doesn't refer to the size of any (sub)network, but to the range of addresses that particular routing table entry applies to. Usually the two are the same (because you route a network or subnet as a unit, right?), but macOS does things a little different for other hosts on the same local network. Let me add some lines before the ones you quoted:

Destination        Gateway            Flags        Refs      Use   Netif Expire
default            openwrt.lan        UGSc           10        0     en0
...
192.168.1          link#4             UCS             2        0     en0
192.168.1.1/32     link#4             UCS             2        0     en0
openwrt.lan        46:94:fc:63:fc:7   UHLWIir        11     3610     en0   1200
192.168.1.125/32   link#4             UCS             2        0     en0

Note that 192.168.1 (short for 192.168.1.0/24) is routed over en0 (aka link#4); not via any gateway, just over the interface itself. This is the network that the Mac itself is on. 192.168.1.1 and 192.168.1.125 are both specific addresses within that network range. If you compare those /32 entries with the 192.168.1 entry, they're basically redundant duplicates; they say the same thing, just about specific addresses instead of the entire network range.

I don't know why macOS creates these redundant address-specific entries, but it's probably related to another thing you can see in the listing above: macOS lists its ARP table entries in the routing table. The "openwrt.lan" entry above (which I'm pretty sure is actually 192.168.1.1, just listed by name rather than number) says that it's routed via en0 to the MAC address 46:94:fc:63:fc:7.

So what you're seeing in the route listing is a mix of actual network routes (like the "default" and 192.168.1 entries), and per-host entries (the /32 and MAC-targeted entries).

It is just CIDR value. You can learn more in here for CIDR.

TL;DR

A CIDR network address looks like this under IPv4:

192.30.250.00/18

The "192.30.250.0" is the network address itself and the "18" says that the first 18 bits are the network part of the address, leaving the last 14 bits for specific host addresses. subnet-mask