Marines are the basic unit of Terran. Terran will build marines throughout the game (unless they go mech, but bio is the primary composition for all matchups). With upgrades marines have high dps & speed. They're a small unit which means you can fit more dps within a small area. They're ranged, unlike zerglings & zealots. Terran's drop ship, medivacs, can ferry around 8 marines while healing them on the ground

In reality the counter to them is splash damage, though good micro can mitigate that somewhat, & Terran isn't going to stop building marines just because the opponent built some splash damage

In some regards this can be rooted in people expecting StarCraft to be like Age of Empires, where as you climb the tech tree you discard previous tech. StarCraft instead prefers tech to fill out a composition & late tech often provides a support role to earlier tech

https://tl.net/forum/starcraft-2/174912-the-problem-with-mar... (2010)

In the game of Rock, Paper, Scissors, each choice has a counter choice that thoroughly defeats it. If you knew that your opponent was going to reveal a scissors, you would counter with a rock.

In Star Craft, each unit has opposing units that are extra effective against it, so if your opponent had a bunch of marines, you might build a bunch of siege tanks.

Here's a whole article on counters to the marine unit: https://osirissc2guide.com/marine-counter.html

Wordplay on counter. It’s a PVP game where players are constantly complaining about balance and or counters.

A counter is the play that beats a specific strategy. Rock is the counter to Scissors in Rock Paper Scissors.

There’s no counter to marines