That string gives the device’s location on the PCI bus. In “01:00.0”, “01” is the PCI bus number (computers can have multiples PCI buses), “00“ is the device number, and “0” is the function in the device (devices can have multiple functions, separately addressable on the bus).
PCI buses are separate domains, connected by bridges. Devices on one bus can’t communicate directly with devices on another bus, they have to communicate via the bridge. Bridges were unusual in “plain” PCI systems, but there are many bridges and buses in PCI Express systems; each PCI Express slot is its own bus. Functions are used in a variety of situations; for example, to integrate two devices in a single PCI device (I have an ATTO SCSI HBA which does this), or to provide different features using a single PCI device (typically so that multiple device drivers can easily attach to individual features; I have a DVB-T card which does this, exposing its audio, MPEG and IR devices as separate features in the same PCI device).
Some systems have multiple PCI domains, in which case a fourth set of digits appears before the bug number (“0000:01:00.0”).
This is described at the end of the
The name of the slot where the device resides
([domain:]bus:device.function). This tag is always the first
in a record.