Many petrol engined garden tools such as strimmers, chainsaws and brush cutters have two stroke engines.This means you need to fill the tank with two stoke mixture made up at the correct ratio. Different engines run on different mixtures, 25 parts petrol to 1 part two stroke oil is common for garden tools but your tools instruction manual will tell you what ratio you need.
So how do you go about making sure you accurately measure the right amount of petrol and the right amount of oil? One simple solution is to purchase a two stroke mixing bottle. These have markings on the side, fill with petrol to the first mark and then add two stroke oil to the appropriate mark for the required ratio, screw on the cap, shake, and your mixture is ready to pour into your chainsaw, strimmer or whatever.
The video below gives a practical demonstration. They recommend buying from Screwfix but these bottles are widely available.
One thing to note is that in the video they make more than one reference to lawn mowers having two stroke engines, these are rare. Most petrol lawn mowers have four stroke engines and so use neat unleaded petrol.