ARM is a design. A reference. It is a reduced instruction set. The world of the PC has been dominated by Intel and AMD. They have developed powerful chips that have been put in PCs and servers by default. The instruction set is the way in which the silicon chips process data. Intel have used complex instructions that have allowed increasing speed. ARM designed reduced instruction sets. The idea being that an action that occurs with an Intel chip may take one instruction can happen on ARM with multiple instructions. This isn't a new thing. ARM has been around since the 1980s.
Over the years Intel has relied on computers tethered to mains electrical power. Their processors have required more power to execute complex instructions and the higher wattage has created heat. So Intel based PCs have fans and large heat sinks. This suited the world of desktop computers and servers with big power supplies and ventilation.
In contrast ARM chips have low power requirements. ARM don't make chips but license the intellectual property of their designs. A company is free to build an ARM chip as long as they pay a license fee. ARM chips are made by a number of companies including Qualcomm and Apple.
ARM chips, with lower power requirements have become most successful in mobile. The change from large desktop PCs to lower powered smartphones, tablets and ultra thin laptops has left Intel struggling to supply low power chips. However, because most PC applications rely on Intel instruction sets there have been two parallel computing universes. ARM on mobile and Intel in desktop. The latter having the performance and speed to run the heavy duty productivity applications.
Apple have broken through that barrier by putting an ARM processor inside a MacBook. Although people worried that ARM would not deliver on performance the first reviews seem good.
This is not the end of the Windows PC. Windows is still used in business and business has a lot of applications that are not going to run on ARM anytime soon. However, the Apple M1 chip is another move to a new mobile centric world.