Endure is a triggered keyword (introduced in Bloomburrow) that gives you a choice: when a creature with "Endure N" triggers, you either put N +1/+1 counters on it, or create N 1/1 white Spirit creature tokens. You pick one option or the other—you can't split the value between counters and tokens.
The most common mistake is thinking it's mandatory growth like a typical counter ability—it's actually a modal choice each time, so you decide based on the board. Going wide (tokens) protects you against single-target removal, while going tall (counters) is better when you want a beefy threat or have payoffs for counters.
In Commander, Endure shines because the Spirit tokens fuel sacrifice, tokens-matter, and aristocrat strategies, while the counter mode synergizes with proliferate and +1/+1 counter themes. The flexibility means it's rarely a dead ability—just read the board and choose accordingly.