HtH (short for "Hand - to Hand"), also known as melee, is a method of fighting that involves no weapons or powers that can strike the opponent at long range, or do anything other than help you pound the crap out of him. If a fight is specified to be HtH/Melee in the OP, that means the characters can only slug it out physically.
Examples of powers and weapons that aren't allowed in a HtH/Melee fight:
- Energy blasts
- Any kind of projectiles
- Offensive telepathy
- Any kind of "negative touch" where you can harm an opponent by touching them (e.g. freezing, death touch, disintegration, paralysis, etc.). Even though these are technically melee range attacks, they are considered beyond the range of HtH abilities.
- Knives/blades that aren't part of the combatant's body
- Flying (questionable, it could be argued to be allowed)
- Teleportation (questionable, but the argument can be made that, like super speed, all it does is increase your maneuverability, so it could be argued to be allowed)
Examples of powers and weapons that are allowed in a HtH/Melee fight:
- Super Strength
- Super Speed (although sometimes it is disallowed, but by default it's accepted)
- Martial arts that use pressure points on the opponent's body (although it might seem to fall under the "negative touch" rule, it's really no other special power than a physical attack, combined with a little knowledge)
- Properties inherent to one of the combatants' bodies that don't give them any other advantage than letting them physically attack more effectively (for example, stretching, blades or other features that are part of the combatant's body, etc.)
Note that these are not anywhere near complete lists, they're just supposed to give you a general guideline. Also, the OP may overrule or amend any of these conditions.