Measures your chain to see if it's time to replace it.
If the 0.75mm measurement won't fit in flush, it's not too worn at all.
If the 0.75mm fits flush, but not the 1.0mm measure, you've got some chain stretch, and should be preparing to replace your chain soon.
If the 1mm measure fits in, your chain is worn out. You need to replace it now, and you may have excess wear on chainring/cassette teeth too (leading to chain skipping and a miserable time when you put a new chain on). So if you're in that boat, be careful with new chains... Definitely measure your chain carefully the 'old fashioned' way with a ruler, and examine all teeth closely for wear.