Proximity attacks depend on the distance between the user and the opponent to activate the specific version. Ryu, for example, has a Far Heavy Punch where he performs a long-ranged straight punch/hook, and a Close Heavy Punch where he strikes with a shorter ranged uppercut.
Proximity attacks tend to have notably different properties. Ryu's Far Heavy Punch has longer range and more damage, but starts up slower and does not special-cancel, while his Close Heavy Punch starts up faster and lets him cancel into a Shoryuken, but cannot be done from further away. Some Target Combos also depend on proximity. Ibuki's Target Combo 4 depends on her close LP, while her Target Combo 5 needs her far LP to activate while both share the same initial followup.
Proximity attacks are mostly removed in Street Fighter V with Target Combos like Shinten not having a minimum or maximum distance to activate. Certain proximity normals in the past were brought back as Unique Attacks, letting them be used from other ranges.