Reveal something when character come near something

Hi, guys I am a Front-end-web-designer, i am new to 3d kinda of stuffs, i was wondering how can we achieve this type of effect
SDU - Future Education Like in this website

I think you can understand what i am trying to say and I apologize for my bad English, hope someone would help me and i want to mention after some research I found about raycasting but seems like it only applies to meshes not to 3d objects/character