Muscles act as pulleys between bones creating levers and movement. Muscles are used in every single action your body performs from swallowing your food to blinking your eyes and snatching 60kg.
Over time our bodies become accustomed to t it's everydays activities, when activities change like moving from sitting infront of the computer all week to gardening on the weekend..... Suddenly getting out if bed isn't that fun on Monday
Muscles are made up of muscle fibres, going deeper inside the fibres you'll find muscle spindles, inside these spindles you'll find sarcomeres, the actual contractile elements of your muscles are actin and myosin filaments. These slide across each other and grip for an easy way to describe it and the result is movement. When we do something different physically that our body is not accustomed to these actin and myosin fibres are disrupted. Local inflamation comes to heal the area and the end result is sore but in the long run stronger muscles
If you are sore don't worry, it means you worked hard. The skills is differentiating DOMS vs injury.