What does it mean "tokens" in this sentence: "People thought leaving tokens and lighting candles in the place was a good idea"?

See a translation

In this context, a "token" is a small object meant to represent a feeling or idea. Usually, you would say a "token of" + something.
token of affection
token of sorrow
token of congratulations
token of support

In this case, since it's obvious they are talking about marking a place where something sad had occurred, the "token" is understood to reflect that.

> At the site of the tragic school bus crash, classmates left flowers, and photos, and other tokens for their lost friends.

In this case, "tokens" would probably be things like stuffed animals and other items the students knew their friends liked, and are understood to be tokens of sorrow.

At a wedding, we exchange rings, which are tokens of commitment.