I'm not sure about this specific case, but they can use uranium-lead dating, looking at its isotopes, to determine stones several million years old. They examine the rock the fossil is in to determine the fossil's age. They also combine that technique with some confirmation techniques: examining the bones of the other animals found with the homonids, knowing the time period those animals were alive; looking at the plants in the same stone layers and knowing those time periods; they also look at the stone tools (or lack thereof) knowing how tools evolved to help confirm the age.

* https://en.wikipedia.org/wiki/Uranium%E2%80%93lead_dating

> A fossil hominin cranium was discovered in mid-Pliocene deltaic strata in the Godaya Valley of the northwestern Woranso-Mille study area in Ethiopia. Here we show that analyses of chemically correlated volcanic layers and the palaeomagnetic stratigraphy, combined with Bayesian modelling of dated tuffs, yield an age range of 3.804 ± 0.013 to 3.777 ± 0.014 million years old (mean ± 1σ) for the deltaic strata and the fossils that they contain.

https://www.nature.com/articles/s41586-019-1514-7

ELI5

They found the bone in the rock. Scientists look at the rock and can tell the rocks age. They assume that the bone is about the same age as the rock it was found inside.

And how did they determine the age of the rock?

There are many methods of determining rock age. A common one is finding signature fossils that are known to only exist between certain ages. Putting the rock in a machine to record its magnetic field and correlating this with pole reversals on earth. Using isotope ratios (not just radiocarbon dating), chemical composition, identifying how the sediment was laid down by rivers or the sea etc.

This may be as close to ELI5 as it can be. https://simple.wikipedia.org/wiki/Radiometric_dating

Scientists look at the bone and can tell the bones age. ((Just kidding))