Imagine that Mortal Kombat as a game runs at a speed of 60 frames per second. A frame is sort of like a still image.
Picture every move in Mortal Kombat as an animation - a video of your character performing their specific attack. Each move can be broken down into three distinct phases; startup, active and recovery. Take for example, Johnny Cage's shadow kick. Startup would be the time or animation it takes for his move to transition to "active". During this startup (which happens to be a short period), Johnny Cage can not hurt the opponent. Once he reaches "active" and extends his foot out, the opponent can be hurt. After this phase, Cage's enters recovery frames, where his foot is extended but he can no longer hurt the opponent.
When an opponent is hit by a move, they go into either block stun or hit stun, pretty self explanatory. Brief periods of time where they can not do anything except remain in the block or hit state (without considering Breakers)
Moves are termed as giving frame advantage (usually on block), when the attacker comes out of recovery animation faster than the defender comes out of block stun. Many chain combos have this property.
Moves are termed "safe", usually implying "safe on block" when if an attacker's move is blocked, they come out of recovery fast enough to block or counter any attack from the defender. Moves that give "frame advantage" on block are safe. However, keep in mind that sometimes, moves that put the attacker at frame disadvantage (often called negative frames) can also be safe. This happens if the defender, can not startup a move fast enough to hit the attacker before they can block or counter. Most specials are usually "unsafe" on block.
