a DNS resolver
When your computer needs the address behind a name, it does not personally negotiate with root servers and TLD servers — it hands the whole errand to a helper called a DNS resolver. Think of a research assistant: you ask one simple question (what is the address for www.example.com?), and the assistant does all the legwork of asking the right offices in the right order, then comes back with a single answer. The resolver is the client side of DNS, working on your behalf.
In practice there are two layers. Inside your device sits a tiny stub resolver, which knows almost nothing except how to forward your question to a configured recursive resolver — typically run by your Internet provider or a public service. The recursive resolver is the workhorse: it walks the tree (root, then TLD, then the zone's authoritative server), collects the answer, caches it, and returns it to your stub. Your computer learns which recursive resolver to use either by manual setting or, very often, automatically via DHCP when it joins a network.
Why it matters: the resolver is where nearly all the real DNS work and caching happens, and it is a major performance and privacy point. Because it caches, the second lookup of a popular name is instant. Because every query you make flows through it, it can see which sites you visit — which is exactly why encrypted transports like DNS over HTTPS and DNS over TLS were created, and why which resolver you trust is a real choice. A common confusion is resolver versus authoritative server: the resolver asks the questions and caches; the authoritative server owns the answers.
Your phone joins a cafe's Wi-Fi and learns a resolver address via DHCP, say 1.1.1.1. From then on every name lookup goes there: you ask once for example.com, and that recursive resolver does the root-then-TLD-then-authoritative walk and hands back the IP.
You ask the resolver one question; it does all the asking for you.
Your device's stub resolver normally does not walk the tree itself — it just forwards to a recursive resolver. So your DNS privacy depends heavily on who runs that resolver and whether the link to it is encrypted.