Hello,
It is for pressurizing (inflate) tyres like chuckey said. It shows various pressure scales (such as Pounds per Square Inch [PSI] and Bar). I assume you have an air compressor as these things are frequently sold together with an air compressor for hobby use. Use it in combination with an air compressor for powering tools, never use a scuba tank or pressurized gas cylinder as a source.
Make sure BEFORE using this tool, that you know the pressure of the product you want to inflate. In case of any doubt, don't use it. Compressed air can contain lots of kJ (or even more). When something explodes that has reasonable volume, you may get severe injury to yourself or people around you.
I onced witnessed an exploding truck tire (be happy to be at safe distance). After heading to the scene, bricks have been blown out of the pedestrian lane.
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Do you want to measure the pressure of a tyre without having an air compressor?