The D2 system involves 3 basic stats (Body, Mind & Spirit, averaging 3 to 4 at starting level) which determine how good you are at stuff. Your enemies have stats too and traps/puzzles/challenges have difficulty ratings.
To resolve a situation you roll (or otherwise produce) the amount of D2 scores represented by your stats to attain a challenge rating or contest against an enemy stat roll.
How you produce these rolls is up to you. I am boring so I just roll D6's and keep anything of 4+ but the recommended method is to paint and varnish beans (fitting with the game theme) with +/ - on each side (or 1/0, tick/cross, boo/hooray or whatever you think is most fun in your group).
It is a great fun game, very silly and a good starting point to introduce kids into the hobby. It is also open-ended enough to make more complex rules if you wanted to. Tunnels & Trolls creator Ken St. Andre loved the game and has written adventures for it.
Hope this helps. Give it a go if you fancy something a bit different one day...
Remember, sometimes you roll a one...