Okay, so do you know what an altar is? It's a special table that people use for praying or making offerings to a god or goddess.
A vedi is like an altar in Hinduism. It's also called a yajna vedi or a fire altar. It's a structure made of bricks, clay or stone. It has a square or rectangular shape and is used for performing yajnas, which are like special ceremonies where people offer things like fruits, grains, or ghee to a divine being.
The vedi has a fire pit in the middle where people make offerings to the divine beings. They light a special fire in the pit and offer things to it to show their devotion and respect. The vedi is decorated with flowers, leaves, and sometimes even fruits.
So all in all, a vedi is a special structure similar to an altar where people perform yajnas and offer things to the divine beings in Hinduism.