What you can load
Paste a PGN straight into the box, or upload a .pgn or .txt file. It handles a single game, a heavily annotated study with variations and commentary, a multi-game file with dozens of games in it, or just a bare FEN string when all you want is a position on a board.
Replaying an annotated game
Moves are laid out as a tree rather than a flat list, so side lines are real branches you can walk into and come back out of. Comments stay attached to the moves they belong to, and NAG annotations — the !, ?, !? marks a commentator leaves behind — are read and shown rather than silently dropped.
Step through with the arrow keys or the navigation buttons, at whatever pace you want.
Analysing it yourself
The board is not read-only. At any point in the game you can pick up a piece and play a different move to see where it leads, then step back to the game as it was actually played. Set up a position from scratch with a FEN string and play from there if you would rather start from a specific structure than from a whole game.
Getting it back out
Export writes the whole tree back to PGN — your added lines included — and downloads it as a file, so the analysis you did here does not get stranded here. No account is needed for any of this: open the page, paste, read.