an access network
Before you can drive on the highway you need a driveway and a local road to reach the on-ramp. An access network is that on-ramp for data: the network that physically connects an end system at the edge to the first router of the wider Internet. It is your first hop — the link and gear that get you from your device into your ISP's network.
Access networks come in many flavors. Home users connect over cable (the old TV coax), DSL (the phone line), or fiber to the home; a key detail is that these are often shared among neighbors and the speed differs by direction (download is usually faster than upload, hence asymmetric broadband). Mobile users connect over a cellular access network — radio from your phone to a nearby base station, then into the carrier's core. Offices and campuses typically use Ethernet and Wi-Fi as their access network. In every case the access network bridges the end system to the rest of the Internet.
The access network is the part most likely to limit your everyday experience, because it is usually the slowest and most contended link on the whole path — often called the last mile (or first hop). The well-engineered Internet core may have terabits to spare, but if your home access link is congested or capped, that is your bottleneck. This is also where your ISP measures and bills your usage, and where technologies like fiber upgrades or 5G make the biggest visible difference.
Your phone's 5G link to the nearest tower, or the fiber line into your apartment, is the access network; once your traffic reaches your ISP's first router, you are off the access network and into the core.
The first hop from your device into the Internet — and usually your real bottleneck.
Shared access links (like cable in a neighborhood) explain why your speed can drop in the evening: you contend with neighbors for the same capacity, even though the core is fine.