Entender (Spanish for, “to understand”; and much more common than the other word for the same, comprender) comes from the Latin tendere, “to stretch out.”
From this root, we get the English extend — which is just a form of stretching.
We can also see how stretching became understanding if we remember that, to really understand something, you need to stretch your brain and creativity to the limits.
The t‑n-d root is clear in both words as well. From the same root, we get other similar words that are metaphors for stretching out: intend, to tender, and even tentative