A scissor lift is a machine for lifting you up to work high in the air. They give you a very sturdy platform to work from, surrounded by railings so there's no chance of falling off. The industry term for them is a MEWP, a 'Mobile Elevated Work Platform.' They get their name from the shape of the struts under the work platform, which are hinged and look like sets of scissors connected together.
Scissor lifts vary from small versions you can move around indoors and run on electric motors, through to large, off-road versions for building sites or grounds-keeping which run on diesel, petrol or hybrid engines. Anywhere you need to reach high up and you would traditionally use a ladder, small scaffolding or tower, you can use a lift for access instead. This could be to carry out maintenance inside a shop, changing lights or point of sale signs. Accessing high stock shelves in a stock room or warehouse. Fixing roll-over doors or other industrial equipment that is mounted high up. Maintaining and cleaning guttering. Window cleaning. Inspecting roofs. Trimming trees and bushes, and so on.
The advantages of a scissor lift over ladders or scaffolding is security and speed. With a ladder, there is always going to be a worry that you can fall off it easily whilst concentrating on your work, or if the ladder feet slip you can have a nasty accident. With a scissor lift, you have four wheels and potentially outriggers to plant in to the ground to make it extra secure. That has a very low chance of moving unless you want it to - some let you move the lift while the platform is raised up. Within the platform, you wear a harness which is connected to the railings so even if the worst happens and you fall out, the harness will save you. If you use a tower or scaffolding, these have to be set up. Using a lift, you just drive up, climb on board and you can be in the air in seconds. No messing about, straight on with the job.