When you hear a word “relay”, probably you think of mechanical relay that can be activated with electromagnet inside. All you need to apply some amount of current across relay coil to operate switch. This way you can control large loads with small signals. But there are some disadvantages of using mechanical relays – they aren’t reliable, they wear off and are slow. So more and more they are replaced by electronic relays with no moving parts. These are called Solid State Relays (SSR) and can be purchased as blocks. If you have a pile of various electronics parts lying around, you can build a SSR relay and pay no money as these are quite expensive.
Follow the Solid State Relay schematic provided by electronics-diy and you will be able to understand and DIY Solid State Relay out of distinct parts. Once finished SSR can be attached to your project like other normal relay that has to switch high AC loads.