1 star :- 

Is obviously poor quality. A broken game that offers no enjoyment.

I urge you to reserve 1 star ratings for games that are un-playable and therefore can be neither criticised nor praised.

2 stars :- 

A game in its very early stages of development e.g. unfinished game loop or lacking a story. 

Game Jam time constraints should be considered however and potential should be recognised. Some things may have been implemented well like solid player controls for example.

This should be taken into account when leaving constructive feedback. 

3 stars:-

These games are good, very playable and fun. 

You may find however that some elements don't quite work or the game is not balanced, 

e.g. enemies/A.I. are too difficult or you get stuck at some point and can't continue.

These games should get a mostly positive review. Make sure to leave suggestions on how to improve the game along with info about any bugs you encountered. 

4 stars:-

A really good experience. Solid game design that is very well implemented. 

You may notice minor glitches in the game but nothing major that interrupts the gameplay.

e.g. missing sound effects, graphical glitches or any other small quirks that grab your attention.

5 stars:-

Thoroughly enjoyable and well made. Reserve 5 stars for a game that impressed you from start to finish.

Keep in mind the nature of Game Jams and these games may not be perfect but if the very minor discrepancies didn't bother you at all and you had fun throughout then it's a 5 star game.