I'm an electrician in Pokhara, in Nepal, and the tool I couldn't do my job without is a digital multimeter. It's a small yellow device about the size of a mobile phone, with a screen, a big dial in the middle, and two long wires with metal probes on the end, one red and one black.
I use it to measure electricity, basically. I turn the dial to choose what I'm testing, whether that's voltage, current or resistance, and then touch the probes to different points in a circuit. The screen then shows a number. For example, if someone's lights keep flickering, I can test each socket and switch to find where the problem is, instead of guessing and pulling out wires.
I've been using one for about twelve years, since I started as an apprentice with my uncle. The one I have now is my third. The first was a cheap analogue one with a needle, which my uncle gave me, and I dropped it from a ladder. I bought this one about four years ago, and it's much more accurate.
It's important for two main reasons. First, safety. Before I touch any wire, I test it to make sure it's dead. In my job, one careless moment could kill you, so the multimeter literally keeps me alive. Second, it saves a huge amount of time. Many electrical faults are invisible, and without it I'd spend hours taking things apart. It also helps me explain problems to customers. When I show someone the reading on the screen, they understand why their wiring needs replacing and trust my advice much more.